I found some strange issue. On long run (3 days and higher) my hashrate drops!

Is there some king of memory leak? Cause if i restart rigs speed will increase for 1 day. After 2-3 days of working i see hashrate drops again...

Need auto restart after 24h? Cause i don't touch my RIG's

http://i67.tinypic.com/vpw4te.jpg