GPS compass problem? Prius thinks it's spinning in circles!

Here's a really weird problem.

I recently acquired a 2006 Prius -- package 8, with all the bells and whistles. It has the GPS navigation system with the original map DVD.

While the nav system usually works fine, I've noticed that every so often it seems to lose understanding of where I am and which way I'm heading. The circle representing the car's location may drift around on the map, and the arrow indicating the direction I'm heading begins to rotate in a clockwise direction, usually ending up with the wrong heading. Eventually (minutes or sometimes hours!), this fault seems to correct itself, and the nav system once again shows me in the right place heading in the right direction, but I've also driven for quite a while with the icon spinning and the car apparently drifting around the map.

I don't think it's a problem with the GPS signal. While this is happening, the GPS indicator on the screen is displayed. I don't think it's necessarily an issue with the older map DVD. I expect the system to get confused if I'm driving on roads that have changed since 2005, but this also happens in areas where I know the street layout hasn't been modified. I've been wondering if there's an internal compass in the GPS system that is faulty, but I haven't found any information online that sounds similar to my problem. Any ideas, anyone?