Triton Spellbind pressure/temperature problem?

Hi,
I am having trouble with a 3 year old Triton Spellbind.
We generally have the temperature set at approximately 40c but just recently the water pressure drops after a minute or two followed by the temperature plummetting to 25c followed by it rising to 55c then settling back at 40c. This then happens repeatedly every minute or so.
We live in a very hard water area so I don't know if limescale could be to blame - I've emptied the scale trap and there was no discernable difference.
Although the pressure seems to drop from the shower there seems to be no mains pressure drop. I've also found that the shower runs fine in "Eco" mode.
Is this indicative of a particular part failing that I could replace or is it a case of replacing the whole unit?
Thanks for your time reading this.
Regards, Nathan.