[milbo]

I may have found a strange workaround for this problem. Here is what I do:
While you have the session open, go to the VIEW menu and open the SESSION item. Once open, you'll see you can view the sample and bit rate, as well as the clock source. It is usually set for INTERNAL. Touch the INTERNAL pull down the menu and set it to S/PDIF. You will get an error message. Clear the error message. Now reset it to INTERNAL. You will then get two more error messages. Once you have cleared them, hit PLAY (or the spacebar). My audio immediately begins playing without any error messages. Very strange but I can replicate it 100% of the time. I should probably just upgrade to Leopard.

I'm running PT LE v7.4cs5 on a Macbook Pro Laptop 2.33 Ghz Intel Core 2 Duo with 3GB RAM. I'm also running 10.4.11. Yes, I know its not approved by Digidesign. I'm also using an MBox 1, MBox 2 mini and an MBox 2 Pro depending upon the project. I have no problem with the MBox 1 and MBox mini 2. However, with the MBox 2 Pro I am constantly running into underbuffer error messages.

Good luck and hope this helps.