I'm having similar trouble,i had head/engine rebuilt.No corners cut,after about 2000 miles,,,,,,,,,,,,,,,,when it was left idling for about 10 minutes,it smoked when you blipped the throttle,then cleared straight away.Anyway,head off again,new stem seals,solved problem.Done about another 2000 miles now,and started doing it again.It really really bugs me.Think my next move will be new set of valves and new stem seals.
Do you recommend anywhere to get the right stem seals

Matt