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I don't believe the USB comms would affect the quality as the serial communications have hardware level error checking and software level CRC checking to ensure the data sent is received without corruption.
I have noticed banding too on my 40D but only at higher ISO (eg 1600) and all I can think of is that when you select a higher ISO, you are increasing the gain on the internal amplifiers. If there are any stray fields (eg 50Hz from power leads is a likely source) they may be amplified too.
Power frequency signals can be detected from many meters away and I can only think of this as being a possibility.
Sorry I can help much more.
Darrin...
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