Anyone knows why XFX RX480 hash rate decrease so much after Claymore V12.4 start from fresh?

When I upgrade to V12.5, I found the power consumption is higher while hash rate remain unchanged, so I changed back. However when I watched it for a while, I found the hash rate start from around 300 H/s each card and decrease very fast in a short time, around the second or third hash rate refresh, it decreased to 270 H/s. Btw I used afterburner to control -30% power limit, otherwise my 1000W PSU would not stand the 5 cards running at full strength which is around 1000W.

I think maybe several reasons below:
1. The motherboard is too old? P5Q Pro Turbo with E8400 core 2.
2. The risers are always in hot environment so degenerated after using for around 3 months.
3. Due to limited power limit, the miner cannot run at full speed.

Any advice? Thanks.