ruff idle

What is your question?

The last few days my truck has been idling ruff on a cold start (engine cold), enough to shake the hole truck, it goes away if i rev it up to 2000rpms and hold it tell the motor is warm then it will idle smoothly, i did put fuel from a passable dirty tank in it with some see foam added, not sure if its a fuel problem or just a timing issue, my uncle thinks its a fuel injector problem, any help appreciated thanks. Its a 1988 F150 with the 5.0L (302) 4 speed 4x4