I have a Picco P-0 .085ci engine that starts fine, idles fine when cold, revs-up fine, but once the engine gets hot, towards the second half of the fuel tank, the idle starts to fluctuate up and down every couple seconds. It is also somewhat more likely to lean-out on speed-runs once the engine is fully heated, but it only happens maybe once every 5 speed runs. The crankcase and crankshaft are relatively new, so an air leak through the front seems unlikely; what else might be causing this behavior?

For the record, I'm running it on 30% nitro, with sufficient extra shimming to compensate for the higher-powered fuel, and a hot glowplug because the engine is so tiny a medium glowplug has a tendency to stall the engine when it idles too long.