Nissan Repair: 1989 nissan pickup stumbles under load, nissan pickup truck, distributer cap

Question
Hi, i have a 1989 2.4 4 cylinder d21 z24 5 speed manual nissan pickup truck. I have 232 thousand miles on it. The truck was running great for a long time (off course or i wouldn't be sending this)I was going down the throughway about 75 when the engine felt like it was running out of gas. I floored the pedal and nothing it made it worse. So i pulled over while it still was running and it ideled fine. I tried to take off again but i couldn't give it a lot gas. So i pulled over again and waited a bit. This whole time it still idled perfectly. I could hold the rpms steady at 2 3thousand rpms no problem. So again i tried to take off. I got up to speed again and turned around at the next exit. Awhile after driving it started to do it again so i pumped the gas pedal off and on untill it finally allowed me to accelerate without it acting like it was flloding or running out of gas. I got it home and tried it again the next day. I could start it fine. I can even rev the engine all i want it was smooth as silk. But again as i tried to head down the road it would cut out everytime i hit about 2000 rpms in each gear. So i checked the :
Fuel pressure
Coils
fuel filter
distributer cap-perfectly clean
inspected the crank angle senor it was all perfectly clean
ran the self diagnostic it came up normal
all my vaccum lines are fine
Alternator output was 14 volts
air filter clean
plugs/wires
egr system
i was wondering if it was the throttle positiong sensor. My manual dosent tell me how to test that. The thing that has me puzzled is how well it idles and you can reve the motor its all nice and smooth. what would cause it to idle so well but not allow me to accelerate under a load? Im open to all your suggestions. If you have a list for me to go through ill try all of it. The TBI is pretty new too i picked it up out of another nissan close to 10,000 miles ago and it was running great. So i know it wasn't because of that. Again thank you for your time. Anything would be helpful. Thanks again -Bill  

Answer
Bill,

I would check the fuel pressure while I was driving when the situation occurs.  That is, put the gauge on and route it under the hood and tape it to the windshield and check the pressure when things screw up.  If it is not the pump I would say the distributor and the optical pickup is failing.  They seem to do this around 2000 rpms like you describe.  But, I would check the pressure first.  Also, on the distributor the optical pickup is under the distributor cover under the rotor.  To inspect for oil you have to take the rotor and shaft off and then the two screws.  If there is oil it is defective and will have to be replaced.