worcester 28 cdi boiler

Our Worcester 28cdi boiler is 5 yrs old, works ok on the heating side and generally ok on DHW but has began to show a slight problem which I was hoping someone has come across themselves before. The boiler, while running hot water seems to cut back and the water temp drops usually while I'm in the shower). It does not run totally cold and the problem usually only lasts for about 15/30 seconds before running hot again. This repeats after about 3 minutes. It is not the shower unit as this problem also occurs whilst running the bath. Which bit needs replacing? Thanks..... Eric & Tina :-)
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