My gut feel is that it is a noise issue rather than contention.
First move will be to ascertain if the sync figures and noise margin are stable or changing and if so, is it a gradual change or a sudden step.
Get a copy of Router Stats Lite from http://www.vwlowen.co.uk/internet/files.htm#routerst...
(FREE) ensuring you choose the right version - there are a couple of router specific ones. Install that, set it to monitor Downstream sync speed and SNR at 30 second intervals and then leave it running until early evening. Get a copy of the screen shoot hosted and provide a link. Leave it running for another couple of days and get those graphs too.
M H C
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