First, while I've not installed on, the plans and perhaps still available heater 'kit' that I think Ryan used to have from Scaled Instruments for the old VP or VP2 that used essentially the same unit, should work. I think there were several plans or implementations that made this a heated gauge, using a heater for reptile aquariums or even a couple large resisters and application of the insulation. Not perfect, but better than nothing.
Second, it would make sense that the build up of fine material brought down by rain would decrease the holding volume of the clean tipper, and thus tip too soon, and would cause over reporting. It would be interesting to see if a gentle but thorough cleaning with a q-tip after soaking would restore previous values, or are there other factors in play.