I have changed the default program on Rapture Pro, so I can't test that. But I have tried in on a few random programs. I didn't notice anything odd.
 
But one program that I made - Karplangor  (http://forum.cakewalk.com/FindPost/3234851) - can exhibit this.
 
It happens because I have 6 very long samples in the 6 elements and each element has 'infinite' polyphony set.
It could take several seconds for each sample to complete, so it won't take many key hits to get hundreds of samples playing at the same time.
 
When this happens the load increases until a dropout occurs and Sonar's audio freezes.
 
If you resume playback, Rapture Pro continues where it left off and has some more dropouts.
 
If you stop pressing new notes and continue playback again and again until the sample playback finishes, you will be back to normal.
 
I don't see this as a bug, it is by design.
 
If it bothers you change the polyphony settings and make them lower (assuming that is the problem you see).
 
I guess you can also set the dropout time to a few seconds...