I use Meldoyne all the time, but on some projects, it is a nightmare.  Maybe someone here we'll know what the factors are that make the difference.
 
In this case, I have a simple project (virtual instruments playing acoustic guitar, flute, bass, and shaker) and three unison vocal tracks.
 
Used Melodyne on the first track.  Vocal was out of sync when project tempo changed.  My fault and I know how to fix that.  Track 1 is OK.
 
Started work on track 2.  Transferred vocal.  After a few eidts, Sonar and Melodyne completely lock up and require reboot.  After reboot, I can make one or two more edits before it locsk up again.
 
Thoughts on what is going wrong?

If you don't know already, when importing a track into Melodyne, latency buffer MUST be set at 1024 and there can be no other plugins on that track (I move them to another track during the import).
 
 

There was also a memory leakage problem in Melodyne if I recall so you need to be using the latest version of Melodyne and Sonar X2 to be on the safe side.

For latency buffer, the only thing I know of is in Preferences | Driver Settings | Asio Panel. That was set to 256 per Sweetwater because of performance issues with Hollywood Strings. I will try 1024. Are you saying I need to re-transfer the track into Melodyne with this value set? Thanks.