Id start by losing the aftermarket swirl pot and sticking a new thermostat in... Alot of ppl recommend an 82degree one from a puma etc... Then drain the system down and refill it with fresh water, run it up to temp and then let it cool down then drain again and refill with fresh coolant and water, run it up and see how it goes.

The ford thermo housing does flow water back to the header tank, the aftermarket one will too, so its both of the i reckon! Try it without the aftermarket swirl pot!