Baxi 105e fault?

My Baxi 105e comes on with the timer, runs until the house is warm, but later if you go upstairs you can see it is starting and stopping, it fires up for a couple of seconds, then goes off for a couple and then fires up again, it carries on like this for ages, and then after a while goes off, but does it again later on.

There are no alarm lights on, just the normal lights. If I turn the heating temp dial down during this, it switches off. I have changed the temp sensor but it still does it. The DHW is working fine and the heating cuts out when the hot tap is run as it should.

Could it be the PCB or is there some valve or switch which could be causing this?

Hope someone can help, thanks.