Its not a head gasket problem, and DONT FLUSH YOUR MOTOR EVER!!! not with bilstien or any 
like that, you will lose compression and shorten the life or your motor. and an engine flush would have nothing to do with this problem.
Why isn't it a HG problem?
It could very easily be one.
And yes, a coolant flush could very easily clear a clog somewhere in the passages. That would indeed solve the problem if that was the case.
he said the radiator is new
And Marlin never said anything about it being an old radiator. Crud from the motor can clog the radiator in a short time, along with an external source.
