intermittent power shower

[rhil00]

Hi All

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?

 

[picasso]

could be a sticking non return valve.

 

[rhil00]

HI Picasso
Thanks for your reply.
I know next to nothing about plumbing.
Is this something I can fix myself, is the sticking non-return valve in the 'bar unit' i.e. the bit with the on/off/hot/cold taps, can I open this up safely? or do I need a professional?