I noticed the same problem with my MeriPlat today in the car driving around. On the Road screen the compass seems to be dead on, but the 'heading' reading is off anywhere from 5 to 100 degrees. The 'heading' measurement is constantly jumping around and not a steady reading off of the compass reading. The GPSr is sitting vertically in my dash mounted retractable cupholder angled slightly towards the driver's seat.
Is the inaccuracy in the 'heading' readout do to it being mounted vertically? I changed it's position, but the GPSr still reacted the same. Is it because it's in a metal vehicle? Why doesn't the metal affect the compass reading? When I got out of the car the 'heading' and compass reading were still of and the 'heading' reading continued to jump around, but not as bad as it was doing in the car.
I have changed the road screen's 'heading' area to 'COG' now. I'll try and get out later and see if it now matches up with the compass, which it should.