Plumbers - Advice Please

I have a radiator in our through lounge that has basically developed a mind of its own. Heats up when it feels like it, goes cold every 3-4 hours, doesnt get as hot as the other rads in the house.

I've tried bleeding it. Water came out straight away so no issue there. Tried balancing, just slightly closed off the valves on all other rads and opened this ones valve slightly more. Finally looked to see if the thermostatic valve was sticking, no issue there the pin is screwing in and out fine.

Totally stumped now.

Any idead?

Thanks in advance.