Temperamental power shower

[squambro]

Hi,
I inherited a power shower when I bought this house, and it is being a bit temperamental.

The basic problem is that it turns on and runs fine, but if you turn it off to soap up (saving water) when you turn it back on it sometimes pauses for a while before letting the water flow as normal. You do not have to turn the 'tap' controller again, so I am assuming that this is not the issue.

This is starting to annoy , as it does not happen every time, just now and then. Any advice gratefully received.