Onyx is a program that cleans up your system, optimizes it, verifies and repairs disk permissions and any disk problems, etc. Titanium Software
There is the link. Its free, and it does work really well. Just make sure you do a back up before you run it.
Have you tried runing BFD off of your boot drive? I run superior drummer on my MBP using Logic 8, and I don't have any problems. I don't know much about BFD2 I must admit.
If all else fails, I would get on that tech support and have them take a look at it.
Just to confirm that you have already tried it without your interface hooked up to your computer.
What about when you load a preset kit. Same thing? I am just throwing out ideas here.
Reinstall BFD2? How full is your boot drive and your external? I know that once you get into the last 20% of your drive or so things can slow down.