Help with gremlins

[1969 C/K CST]

My '84 has me puzzled. At any driving condition, even sitting at idle. The engine rpms will fall to a sluggish pace, then come back up to normal on it's on. This is driving me crazy. I have never had a truck do this. Being an '84 with a guadrajet, I don't know of any electronics. The truck is an '84, bone stock, except for the removal of the emmisions pump. I have drove the truck for 5 years and all of a sudden this starts. I have changed the vacum advance, rebuilt the carb, changed old vacum hoses, and replaced the weird looking goose neck on the intake. I finally don't know what else to look for. I need help before I get to the "go or blow" stage.