My 87 Ford F150 won't idle smooth. It doesn't stop it from operating, but it makes it buck. Any help?

It is running the inline 6 cyl. It mainly has this problem when idling, while driving or sitting still. If I push the accelerator down, it straightens back out. It used to be worse, but I replaced a solenoid on top of the engine and it became manageable. This is getting annoying.