Oil central heating issue

Last year out hot water cylinder in the airing closet had to be replaced due Tom s leak. Ever since the (outdoor oil fired boiler) central heating has had an issue. It fires up fine, runs for about 20 minutes then stops. After a while the switch on the thermostat outside trips and has to be reset before the heating will come on again. I’ve tried adjusting the thermostat outside but it seems to have no effect at all. The radiators also get hotter than I would like.
I’ve had an electrician here who suggested it might be an airlock after the cylinder was drained and replaced. and I’ve had a plumber out who took one look and said that couldn’t be it or the radiators wouldn’t get hot, it,use be the thermostat. When I rang the electrician back he said those were foolproof and early failed. In short theyve both washed their hands and I’m running out of people to call.
Any suggestions?