I've taken two longer cross country flights >3 hrs) lately and have experienced a change in turbo air temp suddenly after ~2 hours flight time.  It has happend four times now, so there seems to be a pattern.

I’ve looked inside the gold-anodized pressurization mixing box and can pass this info along. The box has a Nylon or Delrin slider attached to the hot/cold lever. It selectively covers all or part of the hot and cold inlet tubes as you swing the lever. You can see this function by disconnecting one of the input tubes and looking in the hole—easier with the box off the plane, but it can be done. The other control is simply a shaped plate that selectively covers the smaller side exit tube. When open, this dumps pressurized air through this hole to the back of the engine compartment—probably near the lower left side near the back. In the operation you described, this would be fully closed or you’d not be able to pressurize.

In  considering your analysis so far, I can only explain your symptoms by rotation of the Delrin slider on the hot/cold adjusting lever after a couple of hours of flight. Operating the cockpit control when it’s hot partially, but not fully “resets” the valve on the shaft (it has only a limited travel inside the can) as it slips again during control cycling. When you do this with the airplane cold, it turns back to the approximate correct location and works properly until it gets hot enough to slip again. I don’t recall how the Delrin slider valve is attached to the shaft, but the shaft is attached to the operating lever by a friction clamp around the shaft. Start by tightening this—carefully, as it’s a #4 screw if I recall. If that doesn’t help, dismount the box, open it from the back and try the shaft to slider connection. The first idea is simple to check and easy to fix. The second requires lots more time and effort, including work behind the instrument panel—always a fun task!

Other ideas?

Bob P