Mira Advance Shower - sudden increase in pressure?

We had a Mira shower installed in 2002. It worked fine for some years but then it was noticed that, on some occasions, it ran very cold at high pressure - the pressure was so great it was like a breeze in the room. Thereafter the low pressure indicator would show and the water stopped running.

The Mira rep. has called repeatedly and replaced the same part more than once - a pressure control valve in the shower control cabinet. Thereafter it works fine for a time - but the problem recurs.

On the last call out the Mira rep. commented that he thought we needed a valve to control pressure to the shower. I now understand that this will be disruptive to install and wonder if this is the only - and real - solution?

I have noticed that the shower works fine for about 3 minutes, and then the problem starts - it certainly ensures that you shower quickly! I also perceive that the temperature of the water varies a lot while the shower is working in "proper" mode - almost like a warning of what is to come.

The filter has been cleaned.

Any suggestions welcomed.

Many Thanks,