Ok, so my 351w is acting up i just rebuilt it and when I run the motor the engine overheats but it does it in cycles..(heats up then cools down then heats up then cools down..etc..) it does it fairly quickly, the radiator was 40% clogged so I replaced it entirely and I also put in a new thermostat (yes its in correctly)..I'm kinda stumped here all I have left is the water pump and the guage in the truck.... or maybe there is a blockage in the motor?(however when I took the heads off it didn't look all that bad)

 

heat up - cool down- heat up... sounds like an air pocket trapped inside. Maybe vent off by adding a TEE in the heater hose up on top of the system.

 

I assume you mean a tee fitting ..and I'll probably try that in the mean time do you have any other ideas as to what it might be?