Hate to hear you're having that problem. I have been running Diablo 3 since launch and with the exception of server side issues I have (luckily enough) not had that problem. One thing that I have noticed on the FPS is mine runs between 30-40 at all times no matter what settings I use in game. I run maxed out settings with no issues other then some server side lag every once in a while. I can lower the settings to low on everything and the FPS stays exactly the same. It's strange and I have no idea why it does this. FPS never drops below 30 even with a screen full of mobs and the action gets unbelievably hectic. Hopefully Blizzard can come out with a patch that might fix your problem as it seems as if your computer should be more than capable of playing the game.
Note: Computer specs in sig.
ASUS G74SX-DH73-3D 17.3-Inch 3D Gaming Laptop
Intel Core i7-2670QM Processor
16 GB DDR3 RAM
1.5 TB 7200 rpm Hard Drive
17.3-Inch Screen, Nvidia GTX 560M 3GB GDDR5 Graphics
Windows 7 Home Premium 64-bit