Intermittent Power Shower?

Hi
Was wondering if you could help? Have had a power shower problem for some time now and was browsing this forum to try and fix it, eventually resolved the problem to the following.
House has 4 power showers, 3 are perfect! However 1 operates normally on cold but as you gradually increase the temperature the water strength seems to fall off; the water slows down to a 'mild rain' and then spurts up again to near full strength - cycles every 5-7 seven seconds - and the pump is cutting in/out in tandem.
It's an upstairs shower as is one of the working ones. I've tried switching shower heads but this makes no difference.
Please can you help?

Rhil
September 2009