VAILLANT BOILER (COMBI) F. 1 FAULT, INTERMITTANT HOT WATER?

Hi, we have a Vaillant Combi boiler, about 7 year or so old, it keeps coming up with the fault F. 1 this means if your having a shower it warms up but then after only a minute if that goes stone cold, if you leave it the fault won't go away unless turn boiler off for 10 mins or so, but now even that doesn't work always, it sounds when first turn on as if a pump or something is trying to start up, about three drone noises in a row.

When it's working it's great & would keep working all day if don't turn water off, but more often than not once waters turned off F. 1 fault comes back.

Pressures correct & I've even drained & refilled the system, but still no joy, PLEASE HELP ;0)