my 90 needs help
 

I have a 90 buick century and there is no problem starting it cold but once it has warmed up it stutters and then stalls it i push the gas 1/4 of the way down. after it has warmed up and i turn it off it will not start till it cools down again. i cannot drive faster than 40mph and occasionally as im driving down the road it will just die it starts right back up if i put it in neutral while rolling even if it is warm. i changed the crankshaft position sensor since but it did not correct the issue. im pretty good with cars and i checked the fuel valve located on the fuel rail by letting it spray all the fuel out till it dripped then cranked it and the pressure went right back up. i also took off the idle air control valve and the spring was expanded all the way out and gummed up but i sprayed it with brake clean and got all of the sediment off but it didnt help. i also spray out the intake to make sure it wasnt a stuck throttle and it isnt cuz i removed the honeycomb screen that is located at the front of the intake. im at a loss the only thing i can think is the idle air valve is bad if u have any other suggestions please let me know