>> Probably it was unable to find enough satellites. I think it requires 4 to work out horizontal position(s) but 6 to work out vertical position(s) and in deep, narrow valleys the number of available satellites is likely to be greatly reduced. <<
My GPS (Garmin ForeTrex 201) shows only 4 satellites but usually the altitude it gives is pretty accurate. When this happened, we were not in a narrow valley and it was getting the 4 satellites.

Blame the Americans, bound to be their fault!! Matt.

Probably it was unable to find enough satellites. I think it requires 4 to work out horizontal position(s) but 6 to work out vertical position(s) and in deep, narrow valleys the number of available satellites is likely to be greatly reduced.

Altitude sickness?

I have used the Garmin Foretrex 201 for over a year. Last Sunday, we were hiking around 3,000m (10,000) feet and the altitude on the GPS suddenly started increasing by about 1m/sec. I stopped the GPS and turned it on again and it fixed the problem. BTW yesterday we climbed over 12,300ft and it worked without any problem therefore it is not always crazy in high altitude. Any idea of what could have happened?