I set up a new Windows user account with minimal startup programs (i.e. nothing that uploads or downloads data, like Live Mesh, anything that auto updates, Quickbooks services etc) and am also disabling Microsoft's realtime virus protection when I'm using Sonar. I also set up the Sonar .exe to run as administrator, and disable visual themes when I run it (find all this by right clicking on the Sonar .exe and selecting "properties")
Touch wood, it has seemed more stable so far, in that I didn't suffer any hangs last night anyway.
Of course when virus protection is off I limit my internet use (mainly to this forum!) and remember to turn it back on after using Sonar.