This mainly happens on faster cards, capable of rendering a lot of FPS, much more than what the game is demanding. We've tried using an external FPS cap program, but it caps it at 30 FPS and turns the game very sluggish and with random crashes so we had to disable it.
Overclocked GTX260 here, no sluttering at all. Almost always 60FPS in-game, 30FPS cap in menus (coz it's on flash with 30fps cap, obviously).
Also, have you tried FPSlimiter (info)? Works for DX9/XP only, tho.
