Intermittent errors and no audio and MIDI

[Bergo]

When using PT 9.05
All network is disabled as is virus software. Time machine backup is disabled.

PT will work fine for a while. Then error 6085 buffer underflow reduce buffer size. I do that then an interrupt error occurs saying increase buffer I do that and get 6085 again. I quit restart and all is ok till it happens again.
Just recently I discovered that the problem effects iTunes playback and Sibelius as well. There seems to be no pattern and it happens at random. I can get back audio in iTunes and Sibelius if I go into system preferences and change the clock source away from internal and back again, this does not always work. I am starting to think it maybe the Mbox2 Pro i have as it seems to effect all audio from various sources not just PT. I am sure my cables are in the correct ports ie firewire 400. I am going to try new cables. I looked for a firmware update there doesn't seem to be one for the pro model. I have reinstalled the drivers twice. I have trawled the forums and the error problem has it seems been around for years, with no definitive fix. I am contemplating buying a Mbox3 as a last stand.

Mac Pro 2x2.26 Quad-core Intel Xeon
12 Gig RAM
OSX 10.6.8
Mbox2 Pro
External Firewire LaCie d2 Extreme 160gig recording drive
Protools 9.0.5

[Bergo]

I have got a new FW cable and so far audio and midi is working ok