"Audio device buffer underflowed..." Help rid me of this buffer error

[Jim Digby]

I have been using Pro Tools LE MBox Pro 2, version 7.1.3, with my iMac for the last 14 months, without any real problems.

Suddenly, a few weeks ago, I start getting an error message when I playback or record (mostly recording): "Audio device buffer underflowed. If this occurs frequently, try decreasing the H/W Buffer Size in the Playback Engine or remove other Firewire devices (-6085 or -6086)" . The message sometimes comes within a few seconds of starting a session, or can come up to an hour or so into a session. But each time it occurs the session will then no longer allow me to continue either recording or playing back for very long.

My specs:
iMac Intel 2.16 GHz
OS x 10.4.11 (I had 10.4.9 when this error message started a few weeks ago)
Space: 447.33 GB available

I save my Pro Tools sessions to the internal drive. All my sessions are currently being saved in my documents file, although to be honest I tend to delete them after a short time, when I have had the chance to put the final mix onto a CD or Dat tape.

As for external hardware - other than the MBox going into the FW400 port on the back of my Imac, just a printer via USB and my BT router via its thing. No other FW port is in use.

Things I have tried:

*As it suggests in the message, I have tried decreasing the buffer, plus increasing it, to no avail.
*I bought a new Firewire cable (because a Digidesign engineer told me that "The DAE errors 6086,-6085,-6097 all relate to communication error as a result of a interruption or conflict with the data throughput to the unit."). No joy.
*I talked to an engineer about geting an external drive, but they don't think this could help.
*I reinstalled Pro Tools LE.
*I tried the two following workarounds I found on a website (they are meant to work with version 7.3.1, but worth a try with 7.1.3 I reckoned): "Connecting Mbox 2 Pro after the Mac has booted will prevent this behaviour." and "Quit Pro Tools, disconnect then reconnect the Firewire cable, then re-launch Pro Tools". Neither worked, as sooner or later the pesky buffer message comes up.
*Resetting the PRAM hasn't worked
*Repairng permissioms hasn't worked
*I followed the recommended tips to ensure that my iMAC is fully optimised to run, tweaking settings, deleting the preferences etc.

The engineers tell me that 10.4.11 is not supported with my version of Pro Tools, which I accept. However, this error message was coming up when I had OS X 10.4.9, only a few weeks ago, and is still doing it now that I have 10.4.11 (no change), so I can't see how having 10.4.11 is causing this buffer error.

Quite a wretched problem this is turning out to be, having explored so many trouble shooting ideas already I hope someone here might have a fresh idea that might do the trick!

Thanks in advance,
Jim.