Great advice from all. If its combustion pressure getting through, then it will bubble in the coolant immediately from the leak b4 it even warms up. It may b very little bubbles @ idle but will still b there. At least u can cross that off the list if u dont see that. Have someone help u do a compression check to see if there is a problem with a specific cylinder just to confirm. I agree, its sounds to me like lean carb and timing is off. Remember a stock cam is around 6 degrees btdc @ idle and a performance cam will want 10-12 degrees btdc @ idle average. Many have installed an aftermarket cam @ the stock timing specs. Big no-no. Follow cam company recommendations. Also replace ur radiator cap if u haven't already. The original is probably worn out and wont hold the required pressure to work correctly.