[campbellcj]

I would guess ignition like John hinted; check out the points (if any). Or maybe the dizzy advance is sticking somehow, which might explain why it is kind of an intermittent problem.

skline - it is a race car so certainly could be a high-revving four (race motor). If it's a stocker then there is little point revving much over 5K as the power drops like a brick after that and usually the valves will start to float in the high-5's.