I need a history lesson. What kind of spark plugs have you been using? What kind of gasoline octane? Short, cold engine runs? Car sitting for a few months?

What I am getting at before pulling the head:

Short cold trips with cold spark plugs could result in carbon buildup on the valve edges. This results in a 'stuck' valve, meaning it will not completely close. Depending on the amount of build up, it can be broken off by revving the engine over 3000rpm for an extended period of time, AKA an Italian tune up.
High rpms will allow the valves to rotate and keep the seating surface clean.

If a valve is bent, nothing will help. Same for a broken or damaged piston ring. In either of these 2 cases, the head will have to be removed to find out what is wrong.

If you attempt an Italian tune up, leave the sparkplug connected and the injector connected. You want the possible combustion to burn off any dirt.

Klaus
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Always willing to listen, just not able to take direction.