Well googling around, that does seem to be a problem child
http://forum.brighthand.com/showthread.php?t=224198
http://groups.google.com/group/micro...50de1b3c5c75ac
http://groups.google.com/group/micro...dcd7747406591e
based on a lot of googling, looks like usbfn is all about activesync.
It seems you don't want to change the MemBase setting - is it possible that the default is set not to serial mode, but RNDIS and RNDIS is missing or otherwise borked?
If you put USB back in, but change the setting to USB Serial Mode for ActiveSync, does it become stable? Or, if it is already set that way, does changing it to the other setting make it stable?
MSDN info on USB FC driver
http://msdn2.microsoft.com/en-us/library/ms895495.aspx
and
http://msdn2.microsoft.com/en-us/library/ms923750.aspx
Best of luck!