Id go back to basics.....drain the system...run a hose through to check the circulation...

Then if thats ok fill it back up, run it up with the cap off...keep topping up untill level reached...heater in the car on hot..a couple of fan cycles

Rad cap back on....if that still returns a problem theres possibly something expanding/contracting when a high temp/cooling down???

Could be air trapped and once you turn off its forcing its way back to the top.