I have my GPS WAAS/EGNOS enabled and so far it's been good and showed no zig zagging what so ever! And it's switched on and proved very effective. even when i have been travaling at speed or walking. Unless you have switched it off and done a lot of random zig-zag patterns (maybe the aliens) and turned it on so it can aquire it's position then turn it off again. The change in batteries could be a side effect due to the Garmin GPS, Mine is a Magellan and when ever i have chanaged the batteries on the hill it has shown no difference in the position or any zig-zag in the track log on screen or in memory map. In my theory this could have been caused by the loss and the reacquireing of satalites in the sky. The differnce in the percived position on the ground can be quite large due to the difference between 3 or 4 satalite being in range. This could be the same for the WAAS ones as they are far more advanced that between me and a friend one night i had a EPE of 3 meteres and his was saying that he could have been within 15meters! So perhaps this anomalie could have caused the GPS to do this If yours is WAAS EGNOS enabled i don't know! I dunno maybe it could be wise to phone garmin and ask why erhaps it could be a fault with your transciever, if not it could be something bigger!