I had the same thing happen on both legs of an out-and-back a few weeks back, three days apart, about 2hrs into the flight for both occurrences, and in the same approximate location (between Cheyenne, WY and McCook, NE). I tried everything - turning off all other avionics, repositioning the device to the dash. etc. Nothing fixed it.
Based on it happening in the same spot I wrote it off as 'ground based interference' despite being at 12,500'. No NOTAMs for the GPS constellation (NOTAM ID: KGPS) applicable at the time either.
Hasn't happened before or since.