Virtual dub capture mode lags every 20 seconds on my new monitor

I didn't change any settings. I hooked up my new monitor (23" widescreen from a ~18" standard) and went to capture from my xbox and I get lag every 20 seconds (20 seconds starting when I first see the fps drop). Interesting thing is that I don't get lag if I switch the xbox resolution to 1080i. Anyone understand the problem? Thanks for any help you can give!