Which version of 5.0 are you using? I had similar problems when using using Vegas 5.0c. There was an update to 5.0d available freely on the Vegas site that fixed a few bugs like this. I'm using 7.0 now because even with the updates 5.0 had too many problems (and so did the first few versions of 6). v7 is a lot more stable and you can still import the project files from 5.0.

Also what codec are you using for the basis of your edit? I know it plays up sometimes with a few of the lossless codecs and doesn't like Mpeg2 much either.