Engine rough idle

Your 2000 Frontier only has the one TPS sensor.

Hook it to a obd scan tool and report the codes here or google search the codes. You can try to spray some sensor safe cleaner in the engine air inlet. (Clean the maf sensor).

Here are the 3 engines listed for this year. IGNITION TIMING

2002 Nissan/Datsun Truck Frontier Pickup 2WD 2.4L SFI 4cyl

The Ignition Timing

18-22 DegreesBefore top dead center - When throttle positionsensor disconnected and using suitably programmedscan tool.When adjusting idle speed, set parking brake & chock drive wheels, curb idle speed (750-850 RPM)transmission in neutral.

IGNITION TIMING

2002 Nissan/Datsun Truck Frontier Pickup 2WD 3.3L SFI 6cyl

The Ignition Timing

8-12 DegreesBefore top dead center - throttle position sensorharness disconnected.When adjusting idle speed, set parking brake & chock drive wheels, curb idle speed (700-800 RPM)transmission in neutral.

IGNITION TIMING

2002 Nissan/Datsun Truck Frontier Pickup 2WD 3.3L SFI SC 6cyl

The Ignition Timing

8-12 DegreesBefore top dead center - throttle position sensorharness disconnected.When adjusting idle speed, set parking brake & chockdrive wheels, curb idle speed (700-800 RPM)transmission in neutral.

Bleeding the fuel system is typically only needed on high pressure systems like GDI and Diesel vehicles, and only to get them to start if they run dry once.
If your engine keeps running but the idle speed moves up and down there are many things that can cause that. More information is always helpful. Does it always do it whenever the car is running? Or does it only do it conditionally (IE Only when 1st started in the morning, only after warmed up or getting off the dreeway, only with AC on or at night...describe as much as you can.)
Most newer cars have computer controlled idle strategy that wants to idle the car cold around 1000-1500 rpm (After an initial higher rev called a FLARE) and then bring it down in 3 or 4 minutes to 700ish rpm. If it CANNOT bring it down by normal means, then many resort to something called FUEL CUT IDLE where it actually turns off the fuel momentarily and the RPM will drop because the car is stalling and running out of gas...then back on and it starts to rev back up and when it gets too hiugh it turns back off. Cars that do this can have annoying surges from 500-2000 back and forth at idle forever.

a vacuum leak or MAF meter problem can cause rough idle. ALmost any engine code that turns on CHECK ENGINE lamp may be suspect, does it have any current fault codes?

As well, consider Sticking manifold runners, EGR systems, and temperature sensors all can cause surging, so can a defective Oxygen sensor, slightly jumped timing, and dirty throttle plates the ECM wants to close more but they wont due to dirt. If its not something fairly obvious, it may require deeper checking.

For this types of problem,there are few help inks to troubleshoot the problem.Click the link below to go through the troubleshooting procedure to resolve the issues :----Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html-----------van idles rough? http://howtobyme.blogspot.in/2012/01/2004-ford-freestar-van-idles-rough.html
--------------runs lean and idles rough? http://technoanswers.blogspot.in/2012/02/ford-runs-lean-and-idles-rough.html
-----------Car still runs Rough after tune up? http://howtobyme.blogspot.com/2011/12/car-still-runs-rough-after-tune-up.html
------------Idle flair is noticed on car? Rough idle? car will not idle? Erratic idle speed? http://technoanswers.blogspot.in/2012/02/idle-flair-is-noticed-on-car-rough-idle.html
----------High rpm in neutral? Inconsistant high RPM in neutral? http://technoanswers.blogspot.in/2012/02/high-rpm-in-neutral-inconsistant-high.html
------------Engine Backfires? http://technoanswers.blogspot.in/2012/02/jeep-backfires.html----------- Nissan stalls and bucks on Acceleration?

http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-stalls-and-bucks-on.html

-----------
Nissan Pathfinder Backfires? http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-backfires.html
---------How to adjust rpm? http://schematicsdiagram.blogspot.com/2011/12/how-to-adjust-rpm-on-2009-dodge-charger.html
------------Engine cuts out at 3000 RPM? http://schematicsdiagram.blogspot.com/2011/12/engine-cuts-out-at-3000-rpm.html
----------This will help.Thanks.

if it runs rough only at idle with the AC on then it osunds like a the idle solenoid that causes the vehicle to idle up when the ac is on. if its during the entire RPM range then you might want to check your alternator or battery, perhaps one of them isn't supplying enough power to run the engine, lights and A/C

have the carb re built

this sounds similar to a problem i had. as the rpm went over 2000 the engine would stop for a split second and then kick back in.
after a few days looking my garage finally found the problem was a faulty air mass meter.
this cost me £200 in total to be repaired.

the 2001 altima is what i have and I had the same problem as you. turned out to be a bad head gasket factory used cheap ones for a few years before they found the problem. one way to test this is to get a can of carborator cleaner and spray it around the area where the head meets the block under the intake problem should be around the 4th cylinder or close to driver side. if the head gasket is bad the rough idle will smooth out while you are spraying the head.

Sounds like a possible vaccume hose leak?