2001 frontier very bad miss after running for a while just tuned up code comes up #3 misfire what else can it be?

still has a miss Larry, P0300 does mean misfire condition, but does not mean the coils were bad. Bad plugs,wires, injectors, coils, etc. can through this code.
Check the attached links, troubleshooting,instruction and guides, a lot of good info. Good luck .
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How to Diagnose Misfire Codes P0300 P0306 GM 3 8L

P0300 Pontiac Cylinder Misfire Detected Random Cylinders

code p0300 misfire 2000 2005 other than GXP

http://www.automotiveforums.com/t1066960-code_p0300_help_.html

sure lots, you have misfire. when, say EXACTLY.
when does it and when does it not?
guessing is expensive?
misfires in gear or. parked in park or neutral,
or only driving?
why not do the 60,000 mile tune, why not?
spark plugs , that is top of list. not crank shaft sensors.
1: tune it up. spark parts and filters.
2: keyon, CEL glowing , check engine lamp ? good.
3: start car is it glowing now running, yes, that means SCAN it.
4: best practice would be to drive with scan tool connected OBD2.'
and watch to see if you get P030x misfire codes.
The ECU would have told you the CKP crank shaft sensor is bad or good. the ECU does that on all cars, no need to guess and go broke.
if very intermittent it shows up in OBD2 PENDINGS.
the ecu will report 1 in 10 ckp pulses, missing,(sensitive it is this monitor.

Physics.
Misfires:
1: weak compression on 1 or more cylinders.
2: bad spark, (tuneup time and scan it)
3:bad fueling, scan it. see it way off. (can be vacuum leaks)
4: coolant found in the combustion chamber, spark tips look UGLY. nasty . and repeats. with new plug. not looking is FAIL time.
5: EGR closed. at idle, so if misfire is at idle? , bam.
running the EGR is open, so its not egr, if running ,moving are you?

the above order saves time, and cash.
fuel can burn right with bad compression or spark so fueling is dead last.

misfire is P030x DTC codes.
but only trip when much worse ,
no matter, your seat of pants are smarter than any ECU on earth.
no lie.
ever give it a tune, up ,if not why not?
misfire hot or cold block. or both?
sure under load,
the car has a 60k mile service schedule , ever consider doing that? this stops 90% of all misfires.

Misfire can be , (bad 12vdc electrical power to EFI components)

1: lost compression hot. or cold. or both.
2: bad spark, (tuneup skipped, easy peasy, do it)
3: bad fueling. (vast reasons, but air leaks, bad 02 sensor, bad air meter if opted, low fuel pressure or insane high at 60psi not 30.
never changed the fuel or air filter, yes, that tune up list calls.
just to name a few.

it can also be caused by leaking EGR main valve.
it too is a 50k service point, (clean it)

misfire, has many cause, i have 50 reason list. for get that.
1: bad engine compression, or other major engine issues. mech.
2:bad spark
3: bad fueling. (too much or too little)

it only takes a few tests to find which it is, and in that order.
above assumes, 13.3 to 14.7v at the EFI system at all times running.
and EGR is not leaking.
what does engine light mean.
are you saying the CEL , check (service) engine light glows running? if yes ? why not scan it and report all DTC codes present. P0300 is random misfire. and P0301 is #1 cyl misfire.
as 2 examples.

For engine misfiring problem check the help links mentioned below. Click the link below to go through the troubleshooting procedure :---- Engine misses and truck jumps? Hard to down shift? Issue with high gear only? http://technoanswers.blogspot.in/2012/02/engine-miss-and-truck-jumps-hard-to.html

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Bad miss at idle?

http://technoanswers.blogspot.com/2012/01/bad-miss-at-idle.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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Engine Misfire codes for car?

http://schematicsdiagram.blogspot.com/2011/12/engine-misfire-codes-on-chevrolet.html
Engine Misfire codes?

http://schematicsdiagram.blogspot.com/2011/12/engine-misfire-codes-on-chevrolet.html

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Misfire on cylinder 4?

http://schematicsdiagram.blogspot.com/2011/12/misfire-on-cylinder-4.html

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Oil on spark plugs and car misfires?

http://technoanswers.blogspot.in/2012/02/oil-on-spark-plugs-and-car-misfires.html

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This will help.

Thanks.

For engine misfiring problem check the help links mentioned below. Click the link below to go through the troubleshooting procedure :---- Engine misses and truck jumps? Hard to down shift? Issue with high gear only? http://technoanswers.blogspot.in/2012/02/engine-miss-and-truck-jumps-hard-to.html

-----------
Bad miss at idle?

http://technoanswers.blogspot.com/2012/01/bad-miss-at-idle.html

-----------
While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

------------
Engine Misfire codes for car?

http://schematicsdiagram.blogspot.com/2011/12/engine-misfire-codes-on-chevrolet.html
Engine Misfire codes?

http://schematicsdiagram.blogspot.com/2011/12/engine-misfire-codes-on-chevrolet.html

--------------
Misfire on cylinder 4?

http://schematicsdiagram.blogspot.com/2011/12/misfire-on-cylinder-4.html

-----------
Oil on spark plugs and car misfires?

http://technoanswers.blogspot.in/2012/02/oil-on-spark-plugs-and-car-misfires.html

----------

This will help.

Thanks.

check all spark plug wires and replace. age and wear and tear will cause them to crack and therefore create the misfire in your cylinder causing the engine to run rough

the engine miss fire can cause the other codes to set. you need to repair the miss fire and clear all codes after the miss is repaired then see if the 02 sensor codes return. the miss fire is probably a bad spark plug wire a good tune-up should take care of it ........HAVE A GREAT DAY

you will need spark plugs, ignition wires, cap and rotor. you may also need air filter, there's no fuel filter in this one.
if the number 1 cylinder miss fire then more likely that wire is bad.
just do the tune up first, then check on the MAF sensor later. the tune up may fix every thing