Aqualisa Aquarian thermostatic fluctuating temps?

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?