Hi:
I did some more experimenting. It turns out the the longer your exposure times, and the lesser the number of stcakable frames, the higher the chance of getting this grid-effect on your final images. This ASI is not like my D*K, where I can go for long exposures and capture only a few frames.
Unfortunately, in "solving" this problem I came upon another. There are several rows of dead pixels running diagonally across the chip. I wound up returning the ASI120MC to High Point, and they're sending me another one.
-Ed