Can you please detail your equipment specs and settings? (type of audio interface, Windows level, Driver Mode in Sonar Preferences, etc.).
Also, what happens if you load this particular project in Sonar's 'Safe Mode'?
Here is a link to X3's documentation - and it still works the same in Platinum:
https://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=Troubleshooting.05.html The above link has several suggestions for troubleshooting dropped notes and crashes during playback.
The basic idea with loading a project that is failing - using Safe Mode, is to try to first see if the project will load with you telling it during the Safe Mode dialog NOT to load any effects plugins. It is quite possible one or more of your plugins are somehow causing Sonar to crash or have major problems. By not loading them in, through using the option of skipping the plugin loads, you can at least get into the project to save it off under another name, (to save all of your recorded data), and you can also go through a one-at-a-time load of the project with first a single plugin, then closing without saving, reloading in safe mode and trying 2 plugins, and repeating until you either get the whole project loaded, or you discover a failing plugin.
Check out the link, and provide some more details about your system configuration and specs, as well as any information about this project that is different than others that do not fail.
Does this project have any 32-bit plugins being loaded? This can be problematic. BitBridge is freeware provided by Cakewalk and included in Sonar, and it will automatically load to provide a 32-bit plugin a 'bridge' to be able to run in a 64-bit Sonar installation. IF a particular plugin is failing under BitBridge, there is also a commercially available bridging app called JBridge, which serves the same function as BitBridge - and some folks have better luck with particular 32-bit plugins using JBridge instead of the included BitBridge. (first - figure out if this project loads any 32-bit plugins).
Bob Bone