Yes, I see the video "garbage" too, on some programs but not others.
I think it has to do with information that is normally hidden in the vertical blanking intervals. That information is showing up in the exported video.
In standard-definition NTSC broadcasts, cable, etc., each video frame (or, actually, each of the two fields making up each frame) is separated from the next by a vertical blanking interval which contains sync signals, closed captioning, and other information. In bygone days, TV pictures would sometimes roll up and down the screen, and you could see the black vertical blanking stripes carrying white sync inside them.
Nowadays there is more than just sync in vertical blanking, and so the white stuff flickers. If you can see it, that is. You usually never see it, since TVs hold sync perfectly now.
When NTSC is digitized, as happens for non-digital channels in the TiVo unit, maybe the vertical blanking stuff gets digitized too.
Then, when the transfered .tivo file is exported by Toast, maybe the conversion to H.264 or whatever is picking up the vertical blanking stuff.
A lot of time, it doesn't matter, since the display device typically has "overscan": some number of pixels at the top, bottom, and sides of the screen are cropped off, such that the garbage at the top is hidden.
But for devices that don't overscan, it would be nice if Toast's conversion/export capability would allow you to specify your own cropping.