1994 GMC C1500 truck runs ruff after warmed up

this truck runs fine till it is warmed up then around 1500 rpms it will run rough but if u accelerate its runs fine.also it will idle fine and if u give it gas right up to 2000 rpms or more its fine and goes back to idle fine but if u accelerate it easy to 1500 rpms it will run ruff and u let off it will almost die but never does and then back to smooth idle.any ideas?? thanks