A couple of our lads installed uTorrent on their PCs, and very quickly we started having problems with DNS lookups failing. Eventually I traced it to a NAT problem on our 837 router. The size of the NAT table (from show ip nat statistics) would suddenly jump to about 1500 entries, all UDP, and at that point DNS lookups to external servers would fail and wouldn't work again until the router was reloaded.
Is this sort of thing normal? I use uTorrent at home behind a 837 and I've never encountered a problem. However at work it was quite reproducible. Using a packet sniffer I could actually watch the big burst of UDP packets from the PCs running uTorrent and I could see the NAT table filling up at the same time, and DNS then stop working. I didn't check whether it was just DNS that was affected or whether other UDP traffic was killed too.
I listed the router config. The guilty PCs weren't any of the statically NATed addresses, so they would have been dynamically NATed to the router's address.