The pump on my power shower is playing up and I was looking for some
advice.
What happens is that after about 10-15 minutes use (or two successive
showers say) the flow suddenly goes very hot, loses all pressure and then
stops altogether. The pump seems to have died but then starts to 'twitch'
every five seconds or so, which continues even after the tap has been set
back to off. The only way to stop it twitching is to turn it off at the
plug.

I would check the header tank, as it sounds to me as though it is emptying.

Mine gives the same symptoms when two showers are taken succesively, may I
suggest allowing the tank to refill between showers?

Harv