What a coincidence. I just replaced the t'stat in my m119 engine for the same underheating problem. I suggest the same for your engine.
The best tests for the fan clutch are done when the engine has sat overnight and when fully hot. When started cold you will hear the fan force air at such velocity that is is noticable audibly for a few seconds only. At operating temp throw a fender cover or towel or something to restrict the airflow infront of the a/c condenser/radiator. Allow the engine to run without getting anything fouled in the aux. fans. When you see the engine temp elevated as indicated on the temp gauge (110-115 C) pull the restriction away and accellerate the engine to around 1500-3000 rpm. The sound of rushing air will increase. I think it sounds like an old WW2 aircraft.(sorry not a Messerschmidt). You should notice a momentary rush of air also.
Best to you