Why do you need 60 FPS? your eyes aren't capable of really enjoying the added frame rates above 30 anyway.

Your GPU doesn't have a limiter. if you cap it at 30, it won't try to render anything above that. by setting cap to 60, you will be trying to render at 60, there's nothing telling your processors to stop so it will just push itself to the limit. consequently this bogs down your system and causes the frames to drop. the graphics engine is not optimized, remember.

If you're running fine at 30, leave it at 30.