What's with this Variable Frame Rate!?

My source video is standard interlaced HDV from Sony camcorder DV tape that many of us will have. This is 1440x1080 with pixel ratio 1.33 : 1

On up-scaling to to 1920x1080 with Interlaced Dione: DV 2xFPS (No stabilization), I get 50.000 FPS Constant.

I then choose to add stabilization, and same result (with various stab params.

However, I’ve not worked it out yet but, with the same video source, I’m randomly getting Variable Frame rate.


This is a huge concern as its not handled by editing software. I’m using Vegas 20, (why because I started using Vegas 18 years ago and not going to change now) I know this is also a problem with Premier Pro as well.

Can anyone shed some light of why this seems to randomly happen.