I have had an Aqualisa E99 Aquarian thermostatic shower for over 6 years with no problems. Recently, the shower starts fine and the correct temp. After about a minute the temp suddenly drops to cold. After 2 or 3 seconds it returns to the correct temp. its then OK for 15 seconds and goes cold again for 2 or 3 seconds. This 15 seconds hot 3 seconds cold then repeats until you switch the shower off.
The water from the hot water taps is red hot and constant.
Do you think the cartridge is at fault?

Stuart, October 2013