Not to contradict people with more experience than myself, just thinking out loud here: I don't think it's an issue with retainer clearance. 1) I don't know how that'd affect just 6 valves, 8 maybe with the difference in lift from exhaust and intake sides but not 6. 2) I didn't have any issues the first 100 miles or so, that seems like something that would have an affect from the start. 3) I'm using springs/retainers/guides made for this cam and engine. Is it possible to rule this out for sure without disassembling the heads?