I have a Vista HCX since one month (First GPS) and I have the same problem. I thought I did something wrong until I read this post. This weekend, when I looked at the screen and saw the track was being logged around 100m from where I was I powered it off and on and immediately the track restarted where I really was. There is a gap of 100 in the middle of my track. The operation took 15 seconds so I don't think anything changed in the reception conditions.
I don't know why it happened but the track logging was OK for around 2 hours. I have also seen that the track diverged progressively and not in one sudden move of 100m.This was a street and so easy to detect precisely on the map). It took around 100m to get 100m off, almost like a diagonal in a square. It stayed off constantly by 100m until I switched it off. The rest of the day was OK with no more divergence.