What you are indicating suggests an issue with one or more third-party 32-bit plugins. That seems to be at the root of many similar problems that used to be reported far more frequently, as many folks have switched away from attempting to use 32-bit plugins in a 64-bit Sonar installation.
Many times such a problem means some resource isn't getting freed up - some folks have reported disconnect/reconnect of their audio interface seems to free things up, and others report success by logging their user off and back on, while yet others report that only rebooting their computer seems to fix things.
If you DO have 32-bit plugins you need to use - and are using Cakewalk's included BitBridge for the required bridging of 32-bit plugins to be able to work in 64-bit Sonar, you might consider picking up a commercial bridging application called JBridge - which some folks have had some success with for that situation (and where BitBridge didn't work).
Others sometimes keep a 32-bit version of Sonar installed, for projects where a particular 32-bit plugin is for some reason essential to a particular project. That would eliminate any need for bridging, and may eliminate the conditions causing the problems, BUT then you would not have any access to 64-bit plugins for that project in 32-bit Sonar.
And - many of the 32-bit commercial plugin vendors have created and released free or low-cost updates to their plugins that are 64-bit, and I have found many times that to be the case for plugins I had bought back when 32-bit was all there was for them.
You might be able to find 64-bit plugins you could use to swap out some of the 32-bit plugins causing you headaches - there are lots of good 64-bit ones around these days - even freeware.
So, hope some or all of the above is helpful, and I hope things get smoothed out for you. In the absence of 32-bit 3rd-party plugins, X3e is really very stable, so this will hopefully be only a rare situation for you to have to deal with.
Bob Bone