what may be happening is that you are getting GPS feeds from too few satellites. this happens sometimes when one satellite is somewhat in back of or on top of another. your signal from the satellite with the obstructed view is canceled out by the satellite with the full view, giving you less accurate positioning.

i noticed that when i go under bridges or roadways that overhang the road i am on, the GPS tends to skip a little and show somewhat inaccurate data. this is a short coming of the GPS system being satellite based, with no terrestrial repeating system.

your GPS has to have a clear line-of-sight view of the sky, and then the satellites have to aligned with few to no obstructions amongst themselves for accurate positioning. my GPS will accept signal from up to 12 satellites, where as some of the higher end GPS's will accept signal from up to 16. the more satellites you track from, the more accurate the positioning.