[Jack21]

Some other ideas.

Before starting up, reverse flush radiator from bottom up with garden hose and some clamps. Any trash come out the top?

Reverse flush engine.

Like suggested above, take the thermostat out altogether for some of the diagnostics. leave the rad cap off and check for erratic flow, and gas bubbles. Check rad for cold spots.

Road test it with rad cap just barely caught, i.e. no pressure. Stant has lever actuated rad caps that let you do this without getting a face full of boiling coolant. No thermostat pushes a lot more water through the system. Same problem? If OK, put 160 in, try test again.

If the car was running OK, and just now started acting up, we have either gas displacing water in the coolant, or a blockage in the coolant flow. Eliminate one, and we can pinpoint the other.