U-Verse STB causing slow 5Ghz wifi performance.

The aforementioned router is a Wireless Router, right?

What is happening is that the Gateway routes IPTV multicast traffic down each port that requests it.  When the STB goes online and requests a channel, a multicast stream is begun out that Gateway port, it arrives at the switch and is multicast to all ports (unless the switch has IGMPv3 peeking, anyway) which includes the IPTV STB and your wireless router.  Your STB takes the multicast traffic and displays the video information.  Your Wireless Router receives the multicast traffic and, not knowing whether or not any wireless client is listening for it, sends it out the radio.  However, this is UDP multicast traffic, i.e. not TCP traffic that will be acknowledged and can be retried, so the Wireless Router goes into slow mode, dropping the traffic rate to a crawl.

Does that make sense?

UPDATE:

This is why the suggestion to totally isolate network and IPTV traffic.  When there's not a consumer router involved, there is really no need.  When there is a non-wireless router involved, then you are normally okay, so long as the IPTV traffic doesn't go through the router.  When there is a consumer wireless router/access point involved, then if the IPTV traffic arrives at the router, then you have an issue.

So, do not put a wireless router on the same dumb switch (and by this I mean anywhere in a tree of switches that connects back to the same Gateway port) as an IPTV client.  If your switches are smarter, then you have other options, e.g. VLANs, VLAN with tagging, or IGMPv3 snooping.