need help please with a running problem??

[88 gt]

its a 88 gt 5.0 all stock.heres what its doing,the idle gos up and down(900 to 400 rpms) everytime on start up,even when its warm.no matter what speed im at 20 30 or 40 mph,if i hold it there it acts like it wants to miss or buck on me till i hit the gas about half way down then its fine.when it acts up like that and i push the gas half way or to the floor to get it to stop it will pop through the intake once in a while and when i let it idle too long the rpms will go up and down and sometimes the rpms will stay around 1200 and when that happens ill turn the car off and restart it and the idle gos to normal(900)after it stops going up and down.i noticed when i put it to the floor(wot)it dont pull like it should and sometimes it kicks in like its running on 6 cyls and the other 2 kick in.its like a v6 on nos but you never know when the nos is going to kick in.parts replaced=fuel pump reg,full tuneup,fuel filter,timing chain and every gasket on the motor,Parts cleaned= egr,iac,tb.car has 33 psi of fuel press at idle, jumps up to 40 when i hit the gas and its at 41 with the vac line off.it ran the same way be 4 i replaced all the parts.anyone??maby injectors???