hot and cold potterton 24 (non eco)?

boilers playing up again.heating comes on within a few saeconds shut down will do this tree or four times before operating correctly.then when it reaches temp it shuts down when it kicks back in it does the same thing.Now its happening with the shower.It runs hot and cold for three or four cycles before it works correctly.please advise.As over the last 10 years have replaced most parts and i,d like to keep it going for as long as possible.