Hi! unfortunately this sounds like a valve on the cylinder head not sealing properly.
If any abnormal noise is heard, possible valve train problem such as a valve spring, but if not, Chances are, there is a problem with most likely an exhaust valve, or a intake valve not sealing properly at a higher rpm. The head must be removed and examined by a machine shop.
Sounds like you have covered all other points, which proves the above correct.
that is not the problem
I will opt out so others can help.