It's not Sonar, in most instances it's older plugins that cause the instability. This unfortunately is the digital paradigm, and one I'm still figuring out how to solve. If I wasn't a broke student, I would have kept my last two computers, so I could access older projects this way...if I wasn't a broke student.
My suggestion is always freeze tracks after finishing a mix, however, this doesn't get you around the buses, and I tried to bounce entire stems and tracks...ugh what a nightmare.
Sonar is pretty rock solid, a couple of gremlins here and there, but humans design tech in their own image meaning, there will always be gremlins.
Ben