I have this issue too. If I start some download which consumes 100% of my link, the miners can't reach the pool and, the hashrate slowsdown and, never come up again automatically. So, I need to stop/start cgminer.
Ive had it again today. It does seem related with bittorrent which was running overnight. Now you might expect the cause to be in the router (some routers do not cope well with bittorrent), but also this time, restarting the router didnt solve it. I had to restart all my instances of cgminer.