I have been using a cat 5 50ft cable with usb extenders for a web cam on the roof. I had used a 6 ft regular usb extension from the camera usb connector to the usb extender. At Fry's I found longer cat 6 shielded cable which I purchased. Subquent testing I found the ptl function of the logtech pro 9000 does not appear as smooth as before. Is there a significant in the specs that a could affect the camera performance? By the way, the picture is fine.
Based on my experience with my Pro 9000, I have found that the camera is very picky about any "preceived" run over the 8 feet the camera comes attached to. It needs to be buffered and USB signal maintained perfectly. I am using a 16 foot repeater cable I bought from Mono Price to get the camera to my iMAC and that has worked great for me. No frame rate issues and no video issues at all for over a year now.
It could be that your 6 feet of unbuffered extension is just a wee bit too long for the camera to be happy with it. I have a friend who is using a Cat5 USB extension system as opposed to a chain of standard repeaters for his VP2 connection, and he is constantly having problems with drops. I keep telling him to just buy a few of the same repeaters I use, and chain them instead.
If you can eliminate that 6 feet of unbuffered extension, I bet it will work better.