My 2000 Nissan Frontier has sputtering problems and the tac goes crazy. changed the fuel pump, sending unit, spark plugs, cap & rotor. what could be wrong?

I would want to monitor fuel pressure as the engine warms up. But it could also be a coil or ignition module problem, even a relay.
Any chance the check engine light is on ?

THROTTLE POSITION SENSOR, MISFIRE, #4 MISFIRE, KNOCK SENSOR, THESE ARE YOUR TROUBLE CODES, PO120,PO300, PO304, PO325, RESPECTIVELY, REPLACE THE TPS @ THROTTLE BODY, REPLACE KNOCK SENSOR , @ ENGINE BLOCK, REPLACE YOUR SPARK PLUGS & PLUG WIRES ! AND YOU MUST ALSO REPLACE BOTH YOUR AIR & FUEL FILTERS, DISTRIBUTOR CAP & ROTOR , INSPECT, MAY NEED TO REPLACE, I WOULD ANYWAY !!!!!!!! ADD SOME GOOD FUEL INJECTOR CLEANER TO TANK, NEXT FUEL UP !

check timing and fuel delivery.

check fuel filter,or fuel injectors, fuel pressure regulator.Also check fuel filler cap.Check exhaust system for plugged catalytic converter,muffler,bent exhaust pipes,etc.

Sounds like you replaced alot of parts for nothing.
The pickup coil in the distributor "fires" the module which grounds the coil and the injectors.
Those components would need to be tested. The coil fires the plugs when the ground is removed by the module or the key switch.

Maybe the spark plugs?

CLOGGED FUEL FILTER OR CLOGGED FUEL INJECTORS.WILL CAUSE SPUTTERING AND MISFIRE.LEAKING EGR VALVE WILL CAUSE ROUGH IDLING SPUTTERING.

pump is in the fuel tank and is part of the sending unit assembly(replaceable seperately). i was told this by a nissan dealer when my nissan was getting the sending unit recall done about a month ago. i asked them if since the sending unit was going to be new if it included a new pump, they said no, its replaceable seperately but it goes in the sending unit assembly.

I'm hoping that you also replaced the ignition rotor, it wears 4 times more than the cap. This can also be related to a problem with your distributor. Noting that this is a 92 model, and that you have extremely high mileage, the distributor shaft bearing may be allowing the rotor to make contact sometimes, but not all of the time due to the clearance difference as it rotates off center a bit.

This is the other reason to replace the Ignition rotor more often than the cap. New rotors extend to be closer to the dist. cap contacts, allowing the spark more ability to jump over to the post & down the wire to the plug.

Cheking the fuel injectors: If you remove the retainer on each injectors electrical connector, as the vehicle is running you can pull one at a time off just long enough to notice a change in performance. NO change, then either the injector is bad, or no spark is getting to the cyliinder. Each time you pull a connector off, you're removing 25% of the power available to a 4 cylinder engine and that should be easily noticeable. If it is an injector, it would need to be removed probably to clean properly but you can remove your fuel filter, fill it about 1/2 the way up with regular Gumout or STP fuel injector cleaner & reinstall. The car may sputter a minute or two but this will provide a concentrated injector cleaning, if they are opening & working at all.

good luck.