The Garmin Nuvi 760 in my truck seems fine, but it cheats because it also uses the road as a reference. However, yesterday it showed me within a few meters of the waypoint I set a year ago which is close enough for my needs.

If there are issues I'll have a better idea when I use my Garmin 60CSx next weekend hiking down a local ravine. I'll track the ravine and then overlay the track onto the software map to get a ground truth.