I assume you've checked the accuracy of the time of day clock on the machine doing the recording. Has the network changed their feed time?
PC clocks are horribly inaccurate. If you're using WinXP, you can have Windows reset the clock using Internet time. The default Windows reset period is one week(!), but it can be changed with a registry entry. I use 4 hours. It's in the registry at:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient\SpecialPollInterval.
The default is 604800 (0x93A80). Change it to 14400 (0x3840). The decimal number is the number of seconds between time checks.
Before you make the registry change, backup the registry, then go to Administrative Tools>Services and stop the W32 Time service. After making the registry change, restart the W32 Time service. That'll make it start checking (and adjusting) every 4 hours. You should then be able to keep the computer time well within 1 second of the rest of the world.
If you're using Win2K or earlier, go to
http://tf.nist.gov/service/its.htm and download Nistime. It'll do the same thing.