You could have a burnt exhaust valve. You could rip it open, lap the old valves, replace the bad one and clean the deposits off them. All the exhaust valves had pitting where they meet the head. Your intake valves should be fine. That's what I'm doing now with the same symptoms. When it's all done, I'm going to see what's causing me to run so lean. 261,000miles on this engine.
Thanks to HAN for making what seemed like a big impossible job - possible (and fun of course).