How can I stop my shower going cold?

If I turn the shower on with any great pressure the hot water will not come through. So then I turn the shower on with little pressure and the hot water will come through. Then sometimes when I'm in the shower it suddenly goes cold and i have to start the whole process again.

I've just had a new caleffi pressure limiting valve put in but it hasn't fixed it.

The shower is upstairs but the same thing happens downstairs with the laundry taps. It doesn't happen to the kitchen which is just meters away from laundry. All the taps are mixer taps.

Please help!

Elaine
April 2009