This is a common problem. You could email AVS support as they are very helpful. You could install Nero Vision as I find that to be more precise.
Alternatively you could check your original files frame rate and make sure that you aren't changing it. You mention NTSC not PAL, so you might be converting from PAL? I pulled this off a site earlier:
"If you want to convert use an all-in-one tool if you want an easy method, DIKO, The Filmmachine, AVI2DVD, VSO DivXToDVD(not perfect pal/ntsc to ntsc/pal)"
That might help when you understand what the writer means.
Anyway, we digress:
"DivxtoDVD was a simple beast. ConvertXtoDVD is somewhat smarter. Whereas DivxtoDVD added duplicate frames to encode 23.976 to 29.970 (causing jerky output), ConvertXtoDVD encodes this material correctly at 23.976 fps, then applies 2:3 pulldown for playback at 29.970. I believe, although this is untested by me, that it now does the same for NTSC -> PAL conversion. I don't convert formats if it can be avoided, and as I can play DVDs back in their native format, I just leave it as auto".
Here is something else that might help: "Virtualdub (or a variant) can easily change the framerate of the video. Audacity or Goldwave (or similar) is needed to change the audio to keep sync. Open the video in virtualdubmpeg2 (for example) and click on Video -> Framerate, then click on Change to [ ] frames per second and enter 25. Save the new avi. However, before doing this, I suggest you save the audio out as uncompressed PCM first, so you can alter it's length as needed".
From reading the above you will see that these issues can take up huge amounts of your time. So the bottom line is, try Nero and just google all your queries about everything else.
Personally, I'm sure your problem is down to frame rate conversion without realigning the sound track and that is the reason it doesn't happen every time. Last thing, download GSpot, install it and with that programme you can see the framerate of and avi file, the codecs used, in fact everything.
Best of luck