Originally Posted by
DiscoMick
I wonder if the service and refill has released corrosion and gunk from inside and it is floating around inside and causing problems.
You might need to empty the radiator again, with the bottom hose off, put a slow flowing hose in the top and let the water circulate for maybe half an hour and hopefully flush the gunk out the bottom. Then refit hose and refill with suitable coolant. Doing this can only help.
How old is the radiator? Is it working OK or nearing replacement?