over cooling

Your thermostat must be screwed up if your temperature is accurate. Unless your temp probe is in the bottom of the radiator.
I agree with YoungGun. First be sure your readings are right. Second, where is your temp probe mounted? Should be in the intake near water outlet. Third take your thermostat out and get a cooking thermometer and a pot of water and test the thermostat opening temp. It should open starting 10 degrees or so below its rating and be all the way open by 10 degrees after its rating.

After that, put a thermostatic clutch fan in so you can get 10 - 15 horse power back.