how can I stop my vokera compact boiler from delivering hot and then cold w?

My vokera boiler keeps running hot and then cold and back to hot again. The boiler is only 18 months old and it has done this before but corrected itself but is back again with a vengance. We can't get a decent bath. It' the same on hot water only and it is really annoying. I am losing faith with this boiler which also seems noisy in action. can anyone please advise.