I know this topic is a little dated but it is either your sound card or just bad luck cuase my copy of Blood does the same thing.
I know I'm not much of a help but I would love to play the last three episodes without slow preformance....

I have the same problem with the skippy levels.
Episode 1 runs good but episode 2 runs like hell and episode 3 & 4 are kind of laggy.
If anybody knows how to fix this please let me know.
By the way are there any hosted blood 2 servers out there?