[Stocker]

Great advice in all above posts. What kind of fan are you using? Years ago when my clutch fan failed, I tried a flex fan and the truck overheated, similar to your description. Went back to a clutch fan and no more problems, and this with a stock 2-core radiator.

One more thing - get an overflow tank if you don't already have one. Aside from not dumping coolant on the ground, your system will always be full, so it can cool better. And you'll need a cap designed for a closed system.