At this time of year the water coming in to the house is a lot colder therefore the shower has to be turned up higher as most basic showers don't have thermostatic control (only anti scald). The temp knob adjusts the flow of water over the heater. This is probarbly the cause. Wait until spring and see if it improves. You could try reducing the flow to the unit slightly but turning down the isolating valve on the cold water supply pipe if there is one fitted.
