I have same problem, when using AMD Proprietary, but Open Source provide better fps and less hiccup.
Here some screenshot with "net_graph 1" showing fps rate.
Proprietary (fglrx-update): https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-frc3/1048632_10201400201389911_1820109522_o.jpg
Open Source (X.Org Gallium 0.4 on AMD BARTS): https://fbcdn-sphotos-h-a.akamaihd.net/hphotos-ak-ash4/1026048_10201400201429912_712539590_o.jpg