hot water for sink/shower temp fluctuates?

I have a Vokera excell 80 SP ---10 years old recently serviced(new diaphragm etc) central heating appears ok its the hot water thats the problem - it comes on and heats up but will go cool and warm up again even though the burner is continually burning during this operation:? I have swiched the heating off and it still happens. Also the same problem when using the shower (and we all know what that can be like) no changes in system and has been flushed using TSSM-4 sludger remeover etc
any ideas - yes a new system has come to mind but if there is a way round it for a few years it would be well received