When I had this problem, the #7 Intake valve was stretched and bent...necessity required putting more than a thousand miles pulling a trailer after the damage. A p
Pennsylvania dealership found a dead plug and zero compression in #7, but it held oil pressure and didn't overheat. When I pulled it down at home, I found this and this. If you look at that intake valve, at about 4 o'clock you can see how it's not concentric to the seat. Sorry, the standing-valve-looking-like-leaning-tower-of-pisa picture is lost. Replaced that valve and seal, slapped it together (carefully) and got 190 PSI in that cylinder. Runs smooth and quiet several thousand miles later. 147K on it. Don't forget new TTY head bolts!