ProTools works better with firewire drivers when the DAE Playback buffer is raised to level 4. To change this, go to SETUPS menu > Playback Engine. First check to make sure it's not set to level 0 (zero). If it is, try raising it back to its default level 2. If it is already at level 2, raise it to level 4.

Also be sure that the firewire hard drive you're using meets the compatibility requirements listed at www.digidesign.com ; specifically being 7200rpm or faster and uses the Oxford 911 Firewire bridge chipset.