I can't think of any thing else it can be. You could try descaling the rose on the shower head to enhance the flow. There should be a screw in the center of the shower head, take out the rose and clean it with a kettle descaler. You can descale the shower but that is more complex. Probably the thermostat is breaking down. Try the merchant for the cost of a replacement thermostat, also check price of new showers depending on the age of your triton.
best policy, February 2011