I was having the same issues and it turned out to be the vx64 vocal strip causing the problem, I reinstalled the plug in and it fixed it.

mleghorn
I think it's caused when I use a certain VSTi. I'll just avoid that VSTi for now and hope the problem goes away with future releases. 

You should submit a problem report. Maybe the bakers can see what is wrong with the VSTi and either change SONAR behavior or contact the vendor and get them to fix the problem. Over the years I have seen both scenarios occur.

To the Original Poster - please please please post back with information on what plugins your project is using, as we may be able to help find a way to address your issues.  There's not much anyone can do for you if we don't have a clue as to what your are running on, and what is loaded into the project.
 
Bob Bone
 

I doesn't seem to matter which VSTi's I use in my instrument tracks. It seems to happen whenever I have more than 30 instrument tracks. Is there an unofficial limit, beyond which X3 becomes unstable?
 
Plugins: Dune 2, Bazille, Synthmaster, Valhalla Room (reverb)
Sonar X3 Studio (latest version)
 
System info:
Alesis pentium i7
12 GB RAM
Windows 8.1
ASIO4ALL, using optical out from soundcard

No, there is no limit
 
Stability is totally dependent on your computer, interface & how you've got it set up.

Do you have an audio interface, or are you using the on board sound card?  What other hardware (keyboard?) are you using.
 
Using onboard sound puts all the A/D conversions directly on the CPU as opposed to a dedicated audio interface which has its own A/D processing power.
 
My understanding is that Vahalla reverb is resource intensive, so you may be pushing the processing limits if you are using multiple instances of it.  If that's the case, you could try routing the tracks that need it to a bus.
 
You can also freeze some of you synths to take some of the processing load off of your system.  If you need to work on them you can unfreeze (thaw?) them make changes and then freeze them again.

I am wondering if there is something in ASIO4ALL that is hitting either some internal limit, or maybe being limited by one of the settings within its control panel.
 
Have you tried looking at the settings in ASIO4ALL?
 
I am reaching/grasping here - It has been forever ago since I have used ASIO4ALL for anything.
 
Bob Bone
 

ASIO4ALL, using optical out from soundcard
 
Problem solved once again. 
Please purchase a proper audio interface if you wish to have a functioning DAW system. 
It is rare that you will have good results using on board audio and asio4all. Sonar is very particular about this. 
 
 

ASIO4ALL is at best a band-aid, and I would advise that a new/used dedicated audio interface be purchased and installed, followed immediately by a complete removal of ASIO4ALL from the computer.
 
There are relatively inexpensive audio interfaces for between $100-$150 that will have decent enough basic capabilities to certainly exceed what you get with the use of ASIO4ALL.  Interfaces in this price range will work fine - just make sure that drivers exist for your version of Windows.  Prices for audio interfaces go up, based on the included features, such as number of inputs and outputs, phantom power, and quality of converters (analog/digital), but some of these more basic audio interfaces are actually quite good.
 
Bob Bone
 

I upgraded to the newest version of asio4all, and everything seems to be working fine. I don't require much from an audio interface except stability, good sound, and low latency. I don't need any of that other stuff or anything with "fire" in the name (whatever that is). I prefer to keep everything in the digital domain as much as possible. The DAC phase happens in my headphone amp (pictured in my avatar). The headphone amp, btw, is the Burson Audio Conductor, oh, and the headphones are HD800s, and the headphone cable is the Cardas Clear. Any questions? :-)
 
The jury is still out on my current fix. I'll keep pounding on it to see if anything breaks. Stay tuned :-)