Nissan Repair: 87 pathfinder issue, certin, break point

Question
Hello,
I have spent a lot of money on my 87' pathfinder  and still have one problem that noone in my area knows how to fix. When you start the truck when its cold it will run for a sec then after a few seconds it cuts out and stalls if you dont hold the gas down. And if you get to a certin rpm and dont move ur foot from the pedal it revs up and down (while in neutral), kinda sounds like ur ur pumping the pedal up and down. If you push the pedal to the floor it cuts right out and dies out. Its a 2.4L 8 spark plug motor. I have tried different throttal  bodies and no go. I was hoping you may have an idea because every1 who seen it my way told me to take it to the nissan dealearship. But thats gonna cost a 19 year old guy a lot of money. If you could help me out that would be A+++ ....
Thanks and Ill be waiting for ur response.  

Answer
Tony,

I would bet the temperature sensor that lets the computer know that the engine is cold and at operating temperature is defective or the enrichment valve is defective.  I would do the temperature sensor first since it is the cheapest.  The enrichment valve is nothing more than an extra injector that richens the mixture.  I think, on your year, there is a break point where they stopped using them.  So, if yours does not have something like and extra injector in the intake to richen the mixture up then it would be the temperature sensor.  Now, there are three sensors and you want to replace the one that drives the signal to the computer and not the ones that run the gauge or fan.