you should know better,than to ask these questions after all its the plumbers fault the previous rad worked perfectly for the last 10yrs and was only changed because they wanted a different rad with a higher output in a different position.......
boiler has reached set temp internally and needs to loose some heat before refiring turning it off does not help the heat,sounds like it is a circulation/dirt issue and external issue,turn it on and leave it run for a couple of hours on 3 or 4 on the stat/dial.
edit. make sure all valves are...