As a longtime BFD2 and FXPansion tester, I recommend:
1. Check the Fxpansion site and make sure you're running the most recent version of BFD2
2. Make sure your "Samples" are not on your system hard drive, and if you're streaming a lot of audio tracks, then you'd definitely want to have them on a dedicated samples drive.
3. Make sure your Samples drive is fast. 7,200 RPM's at minimum.
4. Always bounce with "Fast Bounce" unchecked in Sonar.
Why? The BFD2 content is comprised of 18 channels of audio per voice. If you're playing lots of cymbals with long decays, the number of voices being played at once will really add up fast. That can be EXTREMELY demanding on your hard drive.
If you're using multiple outputs for your BFD2 tracks, then make sure you have Anti-Machine Gun mode turned off in BFD2. This adds velocity randomization, which is normally good, but because Sonar does multiple passes on bounces you could end up with different layers being triggered at the same time, which could create phasing issues. If you're only bouncing a single stereo output, you'll be fine.
Ok, where were we?
5. I see you have 8GBs of RAM. Are you running a 64-bit system with the 64-bit version of BFD2? If so, have you tried using the "Load All To RAM" preference? If you have the available RAM, this is a great way to take hard drives entirely out of the equation.
6. If that doesn't work try 16-bit mode. This reduces the size of the kits, which will help whether or not you're loading all to RAM.
7. Finally, if that doesn't work, try playing with the Stream Buffer and RAM buffer. Smaller values result in a smaller memory footprint but strain the hard drive more, higher values use more RAM but strain the hard drive more.
Anyway, this is probably more than you needed to know, but the above should help.