I have a 1991 Dodge D150 and having a overheating problem. Have a new engine and transmission. This morning it started making its way to the Hot...and the heater would go to hot cold hot cold, over and over. When I got home and open the hood I could touch the Radiator and it was cool and the Engine was cool.... and I'm clubless what the problem is?

 

I'd put a different thermostat in it and try to bleed any air out of the system. Fill it with coolant and pop your coolant temp sensor out to bleed air there. Make sure you don't have a lot of thread tape on it so it still makes contact. Might be a good idea to measure the engine temp with a temp gun and make sure it is actually overheating and it isn't just a messed up sensor.

Derek

 

Start by confirming coolant system is full by parking uphill,steeper the better and check/fill.
then check sensor system.