Triton Opal Chrome 10.5kw Low pressure problem?

I have a triton opal chrome 10.5kw shower, most of the times i use it i have this problem, but not all of the time.

I switch the shower on, and within a few seconds it goes from the hot setting, to the cold setting, by the time the water has cooled down, it goes back to hot setting, then by the time the water has heated up it goes back to the cold setting.
This can happen sometimes for upto 20 mins, i have to leave the shower running before i get into it, which i know is very bad, but it is too infuriating having it happen when i am in the shower.

i recently replaced the showers solenoid coil,
any help would be greatly appreciated
thanx