My shower cuts out after only 20mins use?

I have a Redring Plus 8.5 Extra shower, after 20mins use, it just stops, it doesn't matter what setting it is on. I have to switch it off at the pull cord, wait for 2mins then switch it back on again and it works perfectly, it won't work unless it's off at the pull cord first. I have spoken to a Redring engineer and he was baffled and said to try a)all the connections or b)it could be the solenoid, the on/off switch or the isolator switch. Do you have any ideas about this. I can't just go buying loads of different parts and hope they will solve the problem. I really hope you can shed some light on what's causing this, as my wife is disabled and we need to use the shower daily. Many Thanks. Phill.