Some optimism again!
The dropouts in this cards case seems to depend just on the fact that I was in a place with known bad contact. Now in a place with known good contact there is no dropout. Though it seems like xwlan 2.13 overestimates signalstrenght with this card and reports higher strenght then there is - the reason why I thought it was dropouts.
With xwlan 3.x on the other hand the strenght showed very low in a known good area - and also worked as low - easily loosing connection.
Writing this now connected 20 minutes without problems.