Check carefully your vehicle speed sensor and gyro sensor. I believe these are the primary position inputs, with GPS as secondary.

My theory is that bad/variable speed sensor and gyro inputs can cause all sorts of positional havoc.

So record your normal speed sensor readings for some common speeds (ie 30mph, 50mph, 80mph) and gyro for common directions (n,s,e,w etc) and keep a note in the car.

Next time the nav plays up, quickly switch to service mode and compare sensor readings with your known, good readings that you noted earlier.

PS: helps if you have a navigator with you, please don't crash your car doing this!