First the obvious points :
Any Virus Scanner ?
Have you tried to throw away iTunes with all the Apple stuff ( Apple Mobile whatever... ET wants to phone home...)
To be honest, I don´t think that is is something like that.
What about IRQ settings, memory or other ressource sharing of the Lan onbord chipset or the audio interface with other units ?
Any Firewire devices, Liquidmix, UAD, SSL Duende, TC Powercore etc ?
Your drive specs are crazy, I wouldn´t expect any problem with that unless driven horribly hard.
Is the Echo Layla working stable without Vienna or do the problems only occur when both are at work ?