It does sounds like you need a good electrician to take a look at that. An electric shower should be on a separate breaker on your dis-board take a look and see if anything is labeled up, if it isn't then maybe a bodge job tapped into the ring main or something. They should be fed like an electric oven feed not with standard 3.5 twin and earth.

It could also be a water pressure problem, unlikely but possible. An electric shower will cut out if the pressure drops too low, this is to prevent the water getting heated up too much and burning you when you are stood under it. If there is a problem with this system then maybe that is causing it?

Cheers

Cris.