check poppet valve. You may be losing head pressure from the Poppit out the exhaust. Can't quite figure why this would not show up on a hose test unless you have very high water pressure feeding the engine.
Put the T-stats back in and check the poppit. Spring is probably shot. The poppit is supposed to open at high speed to bleed off excess water pressure. When the spring fails the poppit opens and lets water bypass the t-stat and upper cylinders. Since the poppit is lower than the highest cylinder there's a cylinder that's not getting cooled.
