If your engine starts out idling smoothly?

but then as it gets nice 'n' hot,the idle starts getting lumpy,what could this be? Over-rich mixture? It starts rather too well I would have thought,even in really cold weather . .what else could it be? a distorted head? I have re-done the valve clearances when hot,and if anything they're the opposite of tight. .it's a three cyl 993cc Daihatsu. On the open road the thing goes like a Maserati,which is one reason I don't want to ditch it. Plus it's really good on fuel.