Sounds like bad QoS settings on your router.
Not in my case at least. Note I also restarted both my routers and it made no difference. I dont know if it was a maxing out of bandwidth that triggered the problem but its something I might test later. So far the problem only occurred once, but on both machines.
It might be only a problem of cgminer calculating the hash rate. On the server side (pool) do you see a drop in submited shares?
In my case cgminer reports a higher hash rate (impossible for my GPU) when i unzip a new version and lauch it for the first time.