Strange, I have never experienced such issue - my device always restarts charging when current charge level drops below limit.

But, I haven't used PC suite mode extensively. Nowadays, I provide net access (be it WiFi or GPRS/3G) to Linux or Windoze devices via USB from N900, is using Qt Mobile Hotspot, which, basically, puts N900 as an USB client and starts USB networking with connection sharing (sets itself as gateway for node acting as USB host). In Linux devices (including Pi with Raspbian), it works out of the box, on windoze it requires installing drivers (or windows) as per instruction in Qt Mobile hotspot.

Maybe it would work as a workaround for you? Another plus of above approach is that N900 remains full control over own GSM modem/network connectivity, (acts, basically, as advanced router) so you can put some interesting things on it, like custom port forwarding, firewall, custom masquerading, or whatsnot.

/Estel