Suspect radiator stat valve?

I have a radiator that was always on despite turning down the stat valve. I suspected that the valve mechanism may have been stuck so gave it a tap. The valve now seems to work. However, after an hour and a half or so of the heating been on, there is now a loud banging within the system and the pressure rises on the combi boiler. What then sounds like a relief valve opens and the pressure drops again to normal levels, this cylce then repeats. Just tried turning the valve full on and it seems to have stopped. All the radiators have been bled, as I initially though air couldv'e been introduced with the freeing of the valve. Don't really want the radiator on full now the weather's getting warmer! Any clues/help much appreciated!