how can i fix my Ideal Isar HE24 Combi Boiler?

hi there, ive stumbled across this website as a last resort before i completly loose my sanity.
My boiler is about 8 years old and hadnt given us any problems until the xmas just gone. The radiators and the hot water became piping hot as soon as they were switched on. We joined the Powergen repair scheme which havent given us much joy, theyve put in two new parts (as far as i know) and now the radiators seem to be heating up on their own accord without them being switched on. e.g in 30 mins they came on 5-6 times for 2mins and then switches itself off again. weve had to resort to switching the boiler off during the night and when not in use as its adding to the bill
Is there anyone that can help me?