This is actually a pretty old thread.

Well first off it's not Dot Crawl.

Dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video. It consists of animated checkerboard patterns which appear along vertical color transitions. It results from intermodulation or crosstalk between chrominance and luminance components of the signal, which are imperfectly multiplexed in the frequency domain.

Most cable providers if not all use mpeg2 compression

Let's use the correct terminology.

Macro Blocking

MPEG 2 encoding is a lossy compression based on Discrete Cosine Transformation (DCT). It breaks the image into small rectangular areas called macroblocks. Within these blocks the grid of picture elements (pixels) are encoded to represent their horizontal and vertical video frequencies. It does this so that when it has to throw out some information, it starts with the higher frequencies (finer detail) and works its way down.

Motion vectors is another compression technique that MPEG 2 uses to take advantage of redundant frame-to-frame information. The use of motion vectors allows the amount of DCT compression to be decreased. If there is so much motion that the encoder cannot keep up, it can no longer effectively use the motion vectors, and the amount of compression is increased. Fine detail is eliminated in each of the blocks, and what's left is more of an average. Since each block probably has a different average, it makes a mosaic looking set of squares on the screen. When things slow down, and the encoder can decrease the amount of compression the detail will return.