Looks like you covered the normal culprits already. I would suggest a check for one very uncommon problem before you scrap it, the symptoms match. In 5 years of commercial repair I encountered this problem twice in a week, it was the only time I ever encountered it. When the engine would reach operating temp the valve seat insert would slide out of the block and begin to move with the valve, with predictable results. In both cases I restaked the insert into the block and done a valve job. It is uncommon, but is worth a check before you give up on it.