Wont run at low rpm/ idle, good at rpms over 4000

I just bought a 1990 N/A that claimed by the owner has been sitting for 4 years and when i went to check it out it started right up but it idled rough and it had a major lack of power on acceleration during my test drive. 2 days later i came back to purchase and it wouldnt start, that night the old owner got it started after fighting with it for a little while, drove it around the block and got it warm so i came about an half hour later to check it out it was still warm to the touch and it would start but only with the accelerator fully depressed and it would stay on as long as the pedal was all the way down but it would only stay at 400 rpm barley staying on then after fighting with that for a while it began to not start at all. i replaced the fuel filter no luck.. so i purchased the vehicle anyways i had it towed to my shop and did a fuel pressure test i put the guage inline right after the filter, and cranked it a few times the pressure went up to 71psi (spec) then as soon as i stopped cranking the pressure immediatley went down to about 45-50 psi then slowly proceeded to go all the way down to 0psi. i pulled the pump and it was very rough looking so i replaced it and the assembly oring as instructed. again no luck but now it will start and idle again at 400 barely staying on but doing this everytime i try to start it. but when i start it using starting fluid and rev it high during start i can keep it at high rpms (anything over 2000) no problem so i sat and kept it revving for about 10-15 mins trying to burn up deposites i figured had built up but everytime i let it go below 2000 it trys to shut off or does. i replaced spark plugs (were autolite? changed with ngk) and the drivers side bank all had dark plugs (sign of a rich burn) but the pass side were all perfect burn signs. i did a fuel injection cleaning (professional through intake not fuel tank style) same problem starts and stays 400 full throttle unless boosted with starting fluid from start. i checked my codes im getting a 34 detonation sensor code? not really sure what that could mean looking more into it in tomorrows research. ive also notice what sounds like a small vaccum leak coming from under the fuel pressure regulator (manifold drivers side) but ive had a few people tell me thats normal (they arnt really nissan people though) and i couldnt find it without removing my manifold, any suggestions on how to diagnos that without vaccum? also im getting some oil in my intake tube from my pcv will be replaceing as soon as im able to but im told that wouldnt cause this problem so first things first