What is your cabling setup like? I know I had a similar issue which turned out to be due to a bad cable splitter. Comcast ended up coming out and finding a few redundant splitters in the box in my building. Once they removed a couple, my HD channels cleared up.
If you're using a splitter at all, try removing it and using only 1 feed to see if it clears up at all. If not, then you'll have to continue troubleshooting, but if so, you may be simply maxing out the throughput.
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