Q: I get good GPS data for a minute or two, then a timeout message. What could be wrong?
A: As a test, try restarting Netstumbler. If the GPS data comes back (and then goes away) again, your machine has an infrared port (or other serial device) that shares the same IRQ as the serial port. Try disabling the infrared port from Device Manager.
A: Is it plugged in? If restarting Netstumbler does not appear to fix the problem, check that your serial cable is still connected. The Garmin eTrex and eMap connectors sometimes pop out.

There is a beta driver from D-Link that is also reported to work. I have not tried it myself yet.