Try a mixture of 50% 92 or 91 octane and 50% av gas. This will give you a chance to see if it is octane related.
Don't know where or when you fueled up but starting the first of this month the gasoline in california was dropped from 92 octane to 91 octane thanks to Unocal (try to bocote Unocal, they are driving up prices and made them lower the octane)
Also are your new heads steel or alum?