My B3 units are veritable lemons, I need to restart them every 1/2 hour to keep it over 400gh. My B5 units are all hashing 440gh at stock clocks after 4.5 hours.
Mine was almost doing the same, hashrate dropping as it warmed up, until I dropped the clock speed to 212.5. The hashrate went up slightly to now a steady 425 GH. Perhaps in doing so it kept the temperatures down and that keeps the chips stable.
I have had mine running now for 24 hours without having to restart it every couple of hours.