With the two caps everything is working fine now.
Seems this has also stabilized the readings from the pots.

Thanks alot!!!

Finally a friend of mine has soldered me the adapter. Everything is working fine except the FFB effects. My stick is a brand new "SideWinder Force Feedback Pro" and works perfect under Windows 98. It is the version without the fan (Product ID. 96755-579-0463576-00000). I have flashed the firmware ...

Is there anyone willing to sell such adapter to me?