I have been having this problem with p78 and just upgraded to p98 to see if that fixes it, but no joy, I still have it!
I have a LaCrosse 2315 using HeavyWeather. Whenever vws starts up, it picks up the total rain and puts into rain today. I have to clear it (using settings>reset min and max) before the next rain or the end of the day, or you can see that will totally mess up my rain readings. My Wundergournd station page already has me up to 25.55 inches for the month when 7.61 would be more accurate, not to mention 1848 inches for the year.
Does anyone know a solution or workaround (automated) for this problem? I try to manually fix it daily, but I often forget.