Radiators only luke warm and bolier not running for long enough

[paltibine]

The boiler is a Baxi Combi 133 HE Plus which is quite old now.

The problem is that when the thermostat calls for heat, the pump comes on and the boiler fires up as expected. The flow pipe gets really hot but after about 2 minutes the boiler stops firing and the pump continues to run. After a a few minutes of pumping cold water round the system the pump stops. A few minutes later the pump starts running and the boiler fires up. This cycle seems to repeat while the thermostat demands heat. The return pipe never gets warm.
I've tried adjusting the radiator temperature knob but it seems to make no difference.

There are no fault LEDs showing and I've tried turning the whole thing off and on again but to no avail!

The DHW works fine.

Any ideas please?

 

[Gasguru]

CH return filter blocked and/or diverter valve sticking would be the first items to check..
Dog of a boiler and must be one of the last still running.

 

[paltibine]

CH return filter blocked and/or diverter valve sticking would be the first items to check..
Dog of a boiler and must be one of the last still running.

Thanks for the suggestions.....
It has had several parts replaced over the years but has generally been quite good I would say although it's on it's third main control board now.