This is the EXACT problem I was having with one of my Avalons. Initially I was able to make it mine by changing its IP protocol from static to DHCP (so that it would get an address from my router). Dont know why that worked but it did for a few days then stopped working. I've since disconnected the two hashing modules from the controller and plugged them into another avalon thats wasnt giving me any crap.

The three that I have left working now still give me random grief though. They will either stop hashing or slow to a snails pace. All I have to do is restart cgminer and all is well though.

These things are a PITA.