Installed 8 low voltage lights with a malibu digital 120w timer 2 months ago. The timer does not always work. I'll unplug then power up the system and then the lights turn on. Never had this problem with the old analog timer, which was in operation for +10 years. Any suggestions?

Unplug transformer, change bulbs with low voltage bulbs from local hardware or home center. There are two screws on back of each bulb.
Copy following link for tips to get power pack to last longer, replacement trippers, warranty, troubleshoot. etc
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html
Reason for unplugging transformer: because product manual lists that as best way to change bulbs. The low voltage transformer output is 12Volts and does not carry enough power to cause you harm if you touch bare wire.
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Copy following link and scroll down to identify low voltage timer:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

This particular line of Malibu timer has 1 year limited warranty.
Don't buy digital, photoeye timers for outdoors.
See if you can set timer so lights are ON all the time, then plug power pack into cheap wall timer.

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Malibu 8300 appears to be a low voltage lighting fixture.
You might have the digital power pack???
Copy following link for operation, operation tips, power pack troubleshoot, etc.
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html#digital
If this is not your timer, then add a comment and describe timer in detail.

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Promoters should include useful information. Not just air-head fluff.
Copy following link for actual useful do-it-yourself information such as warranty, troubleshoot, operation tips, manuals.
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Problem is not identified.
Is timer not functioning? Or are instructions confusing?
Add a comment with more information and include what you have tried so far.

Open following link to read through troubleshoot steps
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

Always a good idea to check outlet with cheap circuit analyzer.
And move timer to different outlet to see if difficulties are outlet-specific.
Try timer at neighbor's house.
Having 3 defects in a row is unusual.

There are no repairs for defective timers.

Timer clock motors stop working for many reasons, bad manufacturing of plastic gears and power surge and dirt and reversed polarity among the causes.
Using household plug-in timer is typical solution since clock modules are not sold separately.

Following link gives tips for having low-voltage power pack last longer, in case you decide to buy Intermatic-Malibu from Home Depot:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

I don't have repair for defect timer, except return to Home Depot and exchange for another.
Try unplugging timer to see if will reset. There is no specific reset button on timer.
Test outlet with circuit analyzer to confirm there are no electrical problems.

The label shows advisories: Don't use extension cord, unplug unit when working on bulbs, and plug unit into GFCI protected outlet to protect from surge and short.

Other advisories not shown: Unplug power pack if breaker trips or if power goes out. Wait until power is on before plugging back in. First bulb should be 10 feet from unit. Do not wrap extra cord around power pack. Match bulb wattage and power pack wattage: 200 watt power pack should have 200 watts and not more or less. Replace bulbs as soon as they burn out.
These advisories should not affect initial operation of timer, but are things that shorten life of power packs,

Open following links for resources:
http://waterheatertimer.org/images/Malibu-8100-9045-01-large.jpg
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html#dawn-dusk

More information is needed.

Open following link for Intermatic-Malibu resources:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

You say clock doesn't work.
Add a comment with following information:
Do you have timer with analog dial?
Does dial rotate as time passes?
Does dial keep good time?
Have you put the black and red trippers in place?
Are trippers push all the way down?
Have you set current time by rotating clock dial 2 full turns before rotating dial to current time?

Intermatic/Malibu ML series timers come with 5-year warranty.
Product is covered by warranty.

Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.

1) Amazon sells 600Watt power pack:
http://www.amazon.com/Malibu-8100-9600-01-Voltage-Power-Transformer/sim/B002ZRPM4A/2

2) I bought new 300Watt Malibu power pack at Home Depot about 2 months ago.
Near the lighting section.
My new power pack has electronic time clock, so no more 'slow clock.'
I think Home Depot had 600Watt.
Lowes carries different brand.

3) Manuals for power packs at Malibu lights.com
http://www.malibulights.com/customer_service/owners_manuals.aspx

4) Tips for buying and taking care of power pack:
-Match up wattage of bulbs with wattage of power pack
-15 each X 20 watt bulbs = 300 watts, so install 300Watt power pack
-Installing a 500W power pack on 300W of bulbs will burn out bulbs and power pack.
-Replace bulbs as soon as they burn out or other bulbs will burn out and power pack will burn out.
-When working on bulbs, unplug power pack or it may burn out.
http://www.malibulights.com/guides_support/low_voltage_planning_guide/select_your_power_pack

5) How to install power pack.
Before unplugging old power pack, check which bulbs are not working.
Replace bulbs using phillips screwdriver.
Unplug power pack.
Notice how wires are connected to power pack, write down terminal connections.
Some power packs have three terminals ... extra terminal is for additional photoelectric eye.
Use pliers or screwdriver to disconnect wires.
New power pack has has same terminals.
Plug in power pack.
Read instruction manual before connecting wires.
Save instruction manual because a few years that manual won't be around as products change.

I think the reason the power pack is recommended for outdoor only is 1) It hangs on the wall and is not secured from falling down and injuring child 2) power packs have open terminals that might be violation of local electric code. The voltage on power pack terminals is just 12 volts but you don't want child sticking tongue on it 3) Noise: power packs make humming sound.

Open following for Power Pack troubleshoot checklist and warranty information and other tips to keep power pack working longer:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html
http://waterheatertimer.org/Intermatic-trippers-and-parts.html

Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.