I am told it is a common problem with the heater chamber scaling up, overheating, tripping out and then waming up again.
try removing the shower head (so just a pipe) and running on full cold to blast out any crap from the heater chamber. Then put on max hot, let it trip and go straight to max cold. repeat a few times and you will help clear some of the rubbish out.
try descaling the shower head (improves flow).
worked for us for a bit, although I think the only was is to but a second heater unit and have one clean descaled heater chamber ready.
http://showerdoc.co.uk/shower-spares/gainsborough/gainsborough-9_5SCL.html
ColdShower, February 2009