Sometimes this has to do with the way the disc in encoded. The optimum use of the bits available it to align the image on a 16 line boundry. MPEG works on blocks of 16 lines. If you align the image exactly in the center sometimes the top and bottom of the image are using very little of a 16 row block and that's a lot of wasted bits of data.
It sounds like you are viewing a 2.21:1 or 2.35:1 disc. Do other discs with this higher aspect ration do this?
There is also a option in some Tosh sets that allows you to adjust the theater wide scroll. Menu->Wide->Picture scroll. I wouldn't adjust this if the majority of your discs are fine.
It is not uncommon for sets from the factory to have screen geometry and/or positioning misadjustments, and it is possible that you are seeing this (e.g. the picture is too high).
If this is indeed the problem, it should be possible to fix this in the service menu by adjusting the proper variables. (I am not sure what the names of the variables will be, but they will probably be something like HPOS, VPOS, HIT, and WID).
Having a calibration disc like AVIA or Digital Video Essentials will help if you want to adjust things, since these discs contain test patterns that you can use to get the image centered properly.
CAUTION: don't change anything in the service menu without first writing down all of the original values.
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