As Dski mentioned more info needed:
1) Check all your vac lines - connected, cracked, etc
2) Did you reuse spark plugs or install new?
3) Were all head bolts and external part(s) bolts re-torgued when hot?
Also depending on mileage, if you just re-did the head - you might be pulling oil past the rings and oil fouling spark plugs. I'd be looking at things that expand when warm 1st. (vac hoses, gaskets)