Three Mile Final
Do you know how old a design your Garmin is? It could be 10 years, though I would have expected Garmin to have updated their software since then...
I have a Skymap 2 which has always been dead on - but it has a suction cup aerial which avoids the marginal signal problem of a GPS with a built-in aerial.
When the data was moving about like that, how many satellites were you receiving and what was the signal quality on them?
The bit about roads showing slightly off is, I am convinced, crap map data from Jeppesen. They tend to get the aviation data right but roads and railways aren't as accurate. My GPS is within 10-30m horizontally and perhaps the same vertically (I live close to an OS triangulation point with a known altitude) but I can drive with it down the A23 or whatever and it shows me up to a mile off the road. I don't think Jepp give a **** about this sort of thing though.