Thanks very much for the feed back, alternate air door was not something i had thought of, how ever its shut tight, excellent suggestion though,
Only happens at cruise setting, 2300 rpm or there about and only above 2000 ft , rpm fluctuates up and down 80-100 rpm. pilot feels like its the prop changing pitch, however surly this would happen on ground run if this was the case.
Mag flash over?
these are un-pressurized mags, but will check for venting. its a possibility as this engine is being operated in 35 deg plus ambient temps,
Thanks again
Fleep