well, setting the time zone affect whether your time is affected by the Daylight Saving Time...
Another possibility is that maybe your comp is not in sync with the atomic clock said by Wiens. And Windows don't always check for synchronization. Therefore, when they check for synchronization it may set your comp off by an hour and then when you fixed it, it will be your local time which again out of sync with the atomic clock.
The reason you only see it like once a week, its probably that is the time Windows check for synchronization an automatically change it for you to be in sync with their clock (but off by an hour for your time).
Hope this clarifies things...