Just changed the dry rotted radiator and took out the thermostat. It can run great for a few miles or 20 miles then the needle gets to half way and within a few minutes it starts making noises under the van like its sputtering and going to quit.
I have to stop and wait for it to cool down. No water loss. No smoke or water out the exhaust so I'm wondering that if it runs so well until it overheats could it still be the head gasket? Or could it be a bad catalytic converter building up pressure? Or something else?

Could be either... but have a shop run a Hydrocarbon test on the radiator... the only way combustion products will get into the radiator is a bad head gasket. That's the most serious problem it might be and easiest to confirm or eliminate.

There are other tests for the cat... some of them you can actually take off and look through and see of its blocked... (its a honeycomb you can look through).

Thanks