Weird Cooling Problem

[another69]

I have a SBC with Dart aluminum heads, an electric fan, and aluminum radiator. Pump speed is 1:1 to engine speed. 160 deg. thermostat, fan comes on at 185. When it is above 60 deg. outside the cooling system works great, no issues, very consistent temps, never gets above 190. I have two temperature sensors on the motor, both in the intake right before the thermostat. One on the driver's side, the other on the passenger. When it is cold out (took it out yesterday, about 50 deg. outside), the passenger side will take awhile to warm up, but cycles the fan as it should. The drivers side will run warmer and will get all the way up to 250 (!) then drop to 180 within a minute or so. This happens over and over again. I think it is the thermostat cycling, and the faster the car is moving the worse it is. Passenger side stays about 160 - 180. I think for whatever reason the water flow is really not as balanced as it should be side- side or there is a thermostat issue. This only happens when it is cold out, but it happens EVERY time it is cold out.