Altimeter tracks wrong

Hello,

I'm using my instinct for a while now and one thing really bothers me. I calibrate the altimeter before I go on a bike-ride, usually by giving the altitude.

I noticed some things after this ride connect.garmin.com/.../3901930141:

  1. the tracked elevation especially in an about 90m deep tunnel (around 26.3km; the tunnel has of course no GPS reception) is about 0
  2. the altitude is obviously not correct on e.g. bridges (around 3.59km, the bridge is one gentle slope up and one down, not two)
  3. somehow the watch seems able to track negative altitudes but how it gets to that conclusion is really beyond me (-30m at about 26km)
  4. there was no thunderstorm, freaky weather-shift or the like on that day

My settings for that ride: Auto-Cal is off, Watch-Mode is auto. In that regard I really miss my Suunto (Traverse Alpha) which tracked it way more plausible: http://www.movescount.com/moves/move161985352. And where one could also configure (per activity) that any change to air-pressure is more likely caused by altitude-change (hiking in the mountains) or by a pending weather-change (sea-kayaking).

Any ideas how to fix this?