Has the motor ever been out?

I had a similar problem with a 71 Cuda and a 71 Chrysler Newport.

Both had casting sand left from the factory on the coolant passages.

I'd clean out the radiator one week and all would be fine for about a month. Then the same old thing.

You might also make sure the lower hose isn't collapsing. The newer ones don't have the "spring" inside to prevent this.