If you have clouds turned on, turn them off.
Options -> Video Settings -> Clouds: Off
Advanced OpenGL: Off
View Bobbing: Off
Anisotropic Filtering: Off
Render Distance: (can be reduced to 8-10 chunks, but I'd do that after trying everything else first)
Edit: Another mod that helps some people is Fpsplus. It is one of those "if it works, great, if it doesn't, oh well" sort of things. I can run it just fine along with fastcraft, but it would also be one of the first things to go if I started getting errors.
Brain dead moment. I thought I was loading Fpsplus, but it wasn't actually working. As was pointed out later on in this thread, it's a jarmod and doesn't work on current versions of Forge.
Remember, support can only really be valid for the unmodified mod pack. Adding/Deleting things to your pack pretty much throws a wrench into the support process. What may work for one person completely screws up someone else. So be careful with what you try, write down what you change so you can change it back if necessary, and try the small things first, like turning off clouds.
For me, I keep two copies of the pack. One that I've tweaked, and then a clean version that is straight from the launcher. That way I can compare the two easily, and if I find a bug in one version, I can see if it was fixed with my tweaks, or conversely, if I messed something up that works just fine in the base pack, I can change things back.