Since you identified it as an intermittent problem, and assuming it's a modified engine (?), I'd check to see if there's bind in the throttle linkage or carb shafts, maybe the return spring. Sounds like the throttle plate (s) not shutting all the way. Could be you're on the edge of the octane requirement of the engine too, but the intermittent brings that to question. Timing, and temperature are also potential contributors, but I'd start with the carb/linkage.