Well I made an interesting test: I moved speed buffer from 5 to 15 sec... and my altitude problem was solved!
I do not think that GPS signal strength is the explanation, especially because speed etc.. was working fine. It is in the soft that we must find the soution!
can you tell me more about the influence of buffer on altitude calculation ??
That's probably because the signal gets weaker when you move, especially when you hold it in your hands or put it in a bag while moving. It also depends pretty much on the current satellite constellation, so it might be better or worse next time. There's nothing one can do but trying to place the device in a spot where the signal is not blocked.
On my HTC smartphone RUNGPS works pretty well except for altitude... when I move.
When I swich on the GPS, altitude is identified with a reasonable precision.
BUT
As show on my track: http://www.gps-sport.net/trainings/parapente_13294
Altitude is updated only when I stop moving. As soon as I move the altitude is not updated anymore.