Wacom tablet pressure sensitivity becomes random

[Sara Whitby]

I got my Wacom Intuos tablet working with the calligraphy pen pressure sensitivity, and it seems to work just fine for the first few strokes. Then, the pressure becomes totally randomized and the lines get all crazy and jagged, varying unpredictably in width throughout a stroke. I can get it to work again for a few strokes by turning the pressure sensitivity toggle off and on again or switching presets, but after a few working strokes it does the same thing. Happens on all pressure sensitive presets.