cm-7000 dropped frames on 720p only

[moto19]

Hi All, long time AK lurker with DTV converter box issues.

I'm just comparing my first two boxes, a CM-7000 and a Tivax STB-T8. The CM has a particularly annoying artifact that I'd haven't seen discussed anywhere (for any brand/model box). On 2 of the 5 Chicago stations that transmit 1280x720p (60Hz frame rate) I usually but not always get what looks like very jerky motion artifacts as if I was only getting 1 out of 4 frames (i.e. 15 Hz frame rate). There are no other video artifacts (no pixelization) and the picture is very sharp (on my 1985 27" fine pitch Trintron). The Tivax isn't quite as sharp, but hasn't got a hint of this problem. I get 33 stations at 100% on both units, being about 7 miles from the transmitters. I thought perhaps it might be an RF overload problem or multipath, so I mis-aimed my attic mounted antenna about +/- 30 degrees with no change.

If any one can offer some suggestions (practical or theoretical) i'd apprciate it. Thanks, Jim