1985 Ford Bronco Starting problems

I Just put a new 302 in my 1985 bronco originally fuel injected. In the mornings it idles pretty high (1500 about) until it warms up and it idles down (800-1000) but i'll drive somewhere not even too far usually- but most of the time the condition is more severe when driving hard- and i'll turn the car off go buy something and i'll come back and it will start just fine but then the rpms just die down and it turns off and it once did it at a stoplight too. I can usually tell i'm going to have problems before I turn it off because it idles a little rough and pretty low when I come to a stop. (500 rpms)
i thought it might be the computer causing this but i'm not sure I also thought it might be an idling valve.

and also i'm not sure if this is related or not but if I accelerate hard, when I get up to about 2000-2500 rpms the engine hiccups almost and the rpms "blink" down, my friend described it as it felt like it was having trouble trying to shift from 1st to 2nd. And when I do this at a standstill the engine squeaks loudly so this may be related to a belt problem but i'm not sure because the seem to be just fine