Humm... I just did a bunch of rendering with real stuff and even threw in some midi - all work rendered well.

Not knowing what hardware or other software you have that may be capturing any sound porting, it's difficult to say. But, if I was having this kind of trouble, I'd do the following:
1. Run a full UNIX maintenance cleaning routine by opening up the Terminal (found in Applications/Utilities)
At the prompt, type in (exactly as this shows): sudo periodic daily weekly monthly

That will clean out tons of garbage. If you have a program for maintenance, run that instead.

If it were a Windows machine, I'd say, re-install the program and, maybe in the end, that's what you'll need to do. But, first, I'd check all the setups and one good place to start is to go to the MIDI panel in BIAB and select restore to factory defaults.

Also, you can go to the Activity Monitor and look at everything that's running - Stop any audio stuff that you think may be causing this...