I think it may be the left cintrifical modulator,no seriously,have you ever got it hot? Because im with the other guy,it sounds heat related.

 

The who?

Unless you're refering to something else with a different name, cause I've had a bunch of these motors apart, and never seen a "cintrifical modulator."

 

The cintrifical modulator is right next to the Johnson rod.

Seinfeld FTW!

 

I still think it is a valve guide problem you are having. another possibility would be a lash adjuster (lifter) also.

As far as cams go (Probably not needed IMO)
I have comp stage 2 na cams with an aluminum flywheel in my ffr roadster. The engine has an awesome lope at idle. The lope does gets really annoying in freeway traffic. The engine surges when inching along at idle. I have to slip the clutch or raise the rpms to stop it. The motor does make 365 rear wheel hp though. If I were to do it over I would use the money I spent on head porting and cams to buy a supercharger instead.