I can't blame you for that! Here is the official tomtom response if you are feeling techie:
Investigations yesterday have resulted in a definite root cause for this problem, and as a result, there is now also a good fix scenario that some of you have already uncovered at this point.
First, how do you fix your device when you've ended up in the bad state of 'getting a quick fix, but solidly end up hundreds of meters away from your actual position'?
- remove the cached QuickGPSfix data from your computer (for me, this is: C:\Documents and Settings\behm\My Documents\TomTom\HOME\Downloads\complete\ephemeris ) - remove the entire 'ephemeris' folder
- remove the 'ephem' folder from your device (or the content of the folder). Make sure that you remove both the prediction file itself (packedephemeris.ee) and the timestamp files
- connect your device to Home and download the latest QuickGPSfix file
- disconnect device from pc, maybe you need to do one more suspend/resume
Problem solved