Do you mean it fluctuates when Windows is idle or when it bootup it is desynchronized??
Cause for the latter it is probably the motherboard coin battery that is dead. Need to change it, set back correct time in BIOS, save settings, and restart.It'll then be fine for a few years.
For the former problem... um I never saw that happen before it's crazy :-O Maybe you could try to synchronize the windows time with microsoft servers. For that click on the clock, then modify parameters. Then you go to internet time tab, and click synchronize (with "time.windows.com" servers, I just tried this server and it worked flawless)