Combi Boiler - Running warm & then cold!? ...

Question

Hi,

I've got a Potterton MAIN COMBI 30HE boiler system.

The problem is it is producing warm(ish) water and then goes cold quite quickly and continues like this whilst running the shower/bath.

The Central heating is running fine and the RAD's are piping hot.

The boiler does fire when the hot tap is on - buts its very annoying when the water is cold... The setting is up as high as it can go for the hot water!?

Can anyone advise?? Thank you in advance!
DG