Konradh, of all the features and capabilities of Sonar, the integration with Melodyne is probably one that has changed the most between X1 and the latest Platinum.
The stability for me is night and day, and the integration within Platinum makes Melodyne seem more like a natural part of Sonar's workflow.
I completely understand why you may have reasons to stay as you are, but I hope you can test out the latest versions and realize the difference.
It's probably a good idea to put your woes within the context of X1 whenever posting a thread like this so that people can help without thinking about how things work today, which is vastly different under and above the hood.
FWIW, I never got on well with X1 and mostly stuck with 8.5 until Platinum came along.