try reinstalling the drivers if it still happens try 14.4 drivers but first use the amd uninstall utility to wipe the old
I did the same thing 2 weeks back & upgraded 2 of my rigs OS to fresh Win 8.1 Pro from Win 7 Pro
I first installed the catalyst 14.4 driver to test some other miners (different algo) but then switched back to Claymore's XPM Miner.
I experienced the same throttling with that & about 10-15% drop in mining so I used that AMD uninstall utility & installed 13.12.
Today I tested something else, I removed all my GPUs from one rig first,
Then switched on the mining with only one card directly attached to M.B. 16x slot & it was running very smooth for more than 6 hours (look at the screen shot)
Then I attached one more card to the M.B. another 16x slot & both cards started throttling again ! (I forgot to take screen shot at that time)
Now all 4 cards are mining but with the same throttling.
May be the above scenario indicates that it has something to do with multiple cards mining at the same time !
Claymore can you check this out or give some solution ?