Dave,

That is a strange one. I've had rpm drop from things like blown intake hoses, but the rpm didn't come back until the hose was fixed.

Most of the problems you listed might cause an rpm "wall", but not a repeatable drop. This might not make much sense, but it sounds like something giving under pressure to me. What could "give under pressure", but then return to normal when pressure is removed? Something like a radiator pressure cap, a pop-off valve....an O ring?

 

The other thing I might suspect would be something in the ignition tuning that's triggered at the peak rpm. A timing thing?

See if retarding the ignition changes things.

 

Just SWAGs.

Regards,

John

 

 

Good advice on the timing.  Do it NOW Dave.

 

Tracy