Sluggish performance

[ShineTrance]

I've been getting bad performance on Portal. The game was fine with good FPS until it dropped suddenly and the FPS fluctuated. I had Portal on Boot Camp (Windows XP) and the FPS was stable. On both OSses, options were on high without AA and trilinear filtering.

What is causing this?

Below are my specs;

2.0GHz Intel Core 2 Duo, 2GB DDR3 RAM, NVIDIA GeForce 9400M

[Darkjad]

I'm getting sluggish performance on Portal in general. i7 iMac, 4gb of RAM, 10.6.3 OS. TF2 runs smooth as glass on Boot Camp, L4D2 runs smooth (gets a little hinky with there's lots of smoke, but w/e), but Portal on OS X required me to drop the resolution to 1080p and remove all anti-aliasing just to get an acceptable framerate.