I have a spare filter adapter, so I just spent some time looking at the oil flow passages. It seems entirely possible to me that it could be the issue.

As confirming suspicion, it seems strange that oil pressure is fine when cold, but decreases as the oil warms up - with a corresponding spike in oil temperature - possibly because the thermostatic valve is not allowing full flow into the cooler and back-pressuring the pump, causing pump cavitation and heating the oil more than is otherwise normal.

I don't remember removing the thermostatic valve for cleaning when I cleaned the adapter housing for re-installation..... maybe, maybe not.... so it might be gunked up.

I'll sure try it tomorrow..... might even remove the cooler lines and install the original plugs in case the oil cooler is somehow plugged.

Frank