Guitar Rig has lots of PDC needs... and when you switch through presets the PDC factor is changing on the fly.
Many of the emulation efx use convolution math which can take a long time to process... the better sounding stuff taps the PDC too.
SONAR gets flaky when you require it's PDC feature to actually do it's PDC thing.
It works... but it's flaky.
If you wrap your head around the fact that the PDC makes it flaky then a lot of stuff starts to make sense and you can adapt a work flow that let's you avoid most of SONAR's flaky stability.
I use a SONAR that is 99.99% stable... but I know what SONAR can't do so I don't do it even if the Cakewalk manual says that SONAR supposed to be able to do it.