Baxi 105e. Heating a bit then cant be bothered!

Hi

I have a 3 year old Baxi combi instant 105e with the following problem.

The central heating light comes on followed by the flame light and then within approx 45-60 seconds goes from 30 degs C to 80 degs C and shuts off. All rad valves are open, all rads have been bled and boiler is at 1.5 bar. I have tryed a new temperature sensor for the central heating with no change at all. If left on full heat and fixed in the on position it will run for about a minute at a time then the flame goes out cos it says it at 80 degs and keeps pumping the water around then after about 5-6 mins will re light then shoot upto 80 and shut off again. Rads are only getting warm not hot!! Help me!!!

is system sludged up.

When I drained the system to change the central heating temp sensor lots of black ****e and rusty red stuff and brown stuff came out so I would have to say yes, Do you think flushing the system will do the trick and if so whats the best way to do this. I have a filling loop attached so I can put a constant flow of water in the ch system. Do you have to disconnect a certain rad in the house to let the gungy poo out? Highest rad lowest rad??? Or do you just work your way around and disconect every rad and flush the pipe work for each one as well as the rad?

check filter in return pipe ! its in the isolating valve