Capt John wrote:
Sounds like an ignition problem. It could be the coil, wires, or the
module under the cap.

Distributor bushings worn a bit. Unlikley but possible, causeing the
rotor to flop around a bit and misfiring on the cap contacts,

but thats a long shot.

Here's another:

Could be the valves floating out. at 3500+ rpm those valves have to
open and seat extremely quickly. it's ppossible that a weak valve
spring could cause an intake to stay open momentarily too long causeing
the engine to lose compression on that one (or more) cylinders and
huffing back through the carb. OR a hydrolic lifter is defective and
not holding the pushrod to the valve tight, thus causing the valve to
not open correctly.

just some thoughts.

Tim