If 50BTC is using a large coinbase, you might have been getting struck by the excessive CPU usage limitation in older firmware just like p2pool was. This might manifest as duplicate shares and only show up as rejects at your pool. Most of the recent updates were created to address precisely that problem. Try my latest firmware.
http://ck.kolivas.org/apps/cgminer/avalon/20130814/
Thank you, ckolivas. I'll consider to move to the new firmware as it gets mature. I just read the feedback that new FW are affected by the lower hashrate than 0703.http://ck.kolivas.org/apps/cgminer/avalon/20130814/
Actually I'm very happy with the 20130813 release and further, it's a lot more stable on 4 hash module Avalons.
Two of my machines always ended up hanging showing values '0' for hash rate, temperature and fan speed. I've always blamed this on the FPGA controler board in the past. All my other machines did not do this. Coincidence, the two problem machines were 4 module machines, the others have 3 modules.
I always had to cold restart them using the PDU and this made their average hash rate over a day much lower than sometimes a 3 module machine. Sometimes they would soft restart themselves but flipflop in and out of idling. Only cold restart really helps.
So far 4 or so days without any need for restart...
More testing is required before I draw conclusions but it seems the problem has been fixed.
Give the new release a try, the reported hash rate is cosmetic.