I'm using Vegas 5 and previewing external with a canopus advc-100. Just today it started acting strangely. When it comes to a fade or a transition it locks up on and continues to show the last frame before the fade or transition starts. Also if it runs past the end of the clips on the timeline it locks on the last frame and keeps showing it or if you click the timeline where there isn't a clip, and it should go blank, it keeps on showing the last frame it was on. The monitor shows everything correctly.

Any ideas? Thanks in advance for your time.

Unplug your ADVC, let it cool.
Additionally, be sure that Scene Detection is off in Vegas. If you've been using V5 for a while, it's likely that the ADVC has gotten too hot. I've had this problem occasionally with an ADVC100.