I'd suggest you try different driver versions and different versions of CG miner and see if you can get back to where you were on Win 8.  Although CGMiner 3.0.0 is working well for me, I found 2.10.5 to be the best version previous that I have reverted back to at times over the last few weeks.  Also, given how new AMD 13.4 drivers are, you might want to try 13.1, which was the most recent non-beta version, if I recall correctly.