Thanks for the responses guys.
I didn't mention initially but this is a street car, 477 C.I. with FAST EFI (throttle body). Fuel pressure is 45-47 psi.
Ignition is an MP chrome box, recurved distributor, and a 45kV coil.
The condition started after a couple of years. It's not driven much. Used to pull rock solid up to 6500; I don't push it much beyond that.
I recently read an old article about Dudek's Hemi and he said it wouldn't buzz past 5000 with 160# / 390# springs (with a similar cam). Just got me thinking about the springs.
So, for the ignition check out, can I bypass the ballast for a quick test. Seems I've done that in the past.