I was getting a big drop from over 200Mh to less than 100Mh on my "work" PC. In the end I tracked it down to OPera browser. Nice browser but this rig is CPU limited and it was really causing the miner to chugg. I notice both Firefox (not as much as it used to) and opera can run away with the CPU in Windows.
Been using IE9 for a bit now, according to this,
http://www.thefreelibrary.com/IE9+most+power-efficient+browser%3a+Microsoft.-a0252823130It's the most energy efficient browser. So if you're using mining on your main PC with Windows you may want to try this to see if it helps increase your MH.