Try this .........
Conect the Go to your computer and use Explorer to browse the contents of the TomTom memory (it will show up as a new drive).
Delete anything in the "ephem" folder on the unit. (Leave the folder itself alone)
Then, on your PC itself, go to:
c:\documents and settings\yourname\my documents\TomTom\Home\Downloads\Complete\ephemeris \
and delete anything found in there (probably several sub-folders - delete them all)
Run TomTom "Home" and a new QuickGPSfix file should be offered. Install it.
Disconnect the Go properly from Home using the Device Disconnect icon, before unplugging it.