There might be something up with the software and the DST setting. It might just be better to select "on" or "off" and change it at the time of the year. Probably work better. Mine is still fine, and hasn't changed.
It is possibly the Timex on your computer. Is your time on your computer set to synchronize with Internet Time? That allows for the best chances of times being correct. The setting is default in Vista and available in XP.