I'd bet it's either dist drive and bushing, or the preload on the lifters. Who assembled the engine? Was the oil pump dirve and bushing new when it was fired? I would pull the distribuotr out, and take a screwdriver to the slot on the drive. Wiggle it. If it moves in any direction except straight up and out, you need to replace them both. If the bushing isnt sized properly when it's pressed in, the drive and bushing can wear out in a verys short amount of time. Seen it twice on rebuilt engines assembled by guys who didnt know about the dist drive broaching tool, or who figured the tight fit would loosen up when it ran. He was right. But at 3K miles, the drive and bushing were worn bad/shot. The fac that another dsitribuor did the same at a different rpm is the hint.