1992 Evinrude Intruder150 overheat signal -- can't seem to fix. Need some help!!

I recently purchased a 1989 Quantum 18 1/2 fish-ski boat. The boat has a 1992 Evinrude Intruder 150. When I purchased the boat I was told of the signal but explained that it was a trim issue. I took the boat out and the overheat signal warns and the SLOW kicks in only after 1/4 mile. Just to be safe I had the thermostats, water pump, and impeller all replaced. I took the boat back out and it is still giving the same signal. I then took the boat to a shop and they didn't water test the boat, but they told me that the motor was not low enough and was sucking air causing the motor to overheat. The motor was of course lowered at the shop. I took the boat back out and it went slightly further than normal, but the signal started back up.

Any other suggestions as to why the overheat sensor keeps going off? I have had people tell me bad sensor, VRO pump problem, or a short. What does anyone else think. I appreciate any help. I am terribly frustrated.

Thanks