home heating systems?

I have an 12 yr old Highflow Electronic system, which is working perfectly for hot water. However the heating comes on heats up some of the radiators and then goes off. The thermostat is connected to the boiler as we have checked turning that down and then up which then lights the radiator light on the control panel.

It seems a bit hit and miss on occasions it fires up the heating and then a minute later just stops.

As its coming to winter I'm a bit scared of losing all heating and concerned that as its intermittent the fault will be hard to find? Two heating engineers have left it working perfectly after 30 mins but its after that it stops working.

I don't want to have to keep paying for engineers to come out and replace parts if the easiest option is to buy a new boiler.
Any ideas