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?


I can see why the pump would suddenly 'die' if the tank was emptying but
why
would it twitch?

It "twitches" because as the incoming water goes into the header tank, and
enough goes down into the pump for it to push some through.

Harv