Whenever I switch to a new version of Sonar, I try to leave projects from prior versions to be edited in whatever version I created them in, so for X3 there are only brand new X3 projects there.
 
I am not saying that is required - I just have no issues with doing things that way so I just keep doing what works.
 
I am on X3d, and Windows 8, but not yet on 8.1, and I almost always use only 64-bit plugins for 64-bit projects.  MinimogueVA and ARP2600VA are about the only 32-bit ones I bring in - using BitBridge.
 
Even with that - the MinimogueVA is starting to get a little flaky - sometimes giving me BitBridge failure messages and other weirdness - so I am afraid that is going to be sent back to 32-bit Sonar only shortly.
 
Bob Bone
 

The only crash I've had in X3 has been related to V-Vocal
 
Apart from that it appears to be rock solid

I haven't had any issues to speak of since X3 was released, and just ran it last night with 10 instances of Nebula 3 Free on a single track.    

Jackel
X3a, b, c, & d have all been completely unusable to me.  I have been using Sonar since Sonar 4 and never really had any major issues with it.  X3 not only crashes itself but makes my entire system unstable.  If I load X3 it crashes quite soon for a who knows what reason at whatever time.  From there if I don't reboot my system other programs start to crash.
 
in case you think I just have a jacked up system... X2 still runs like a charm.  Of course that's what I am using but having paid for the upgrade I was kinda hoping to use it.  At this point I would like my money back.
 
Jeff

I am in the same boat as you. I have been using various versions of cakewalk, for 10 years, and have never had any problems. X3 as been a total failure for me, but X2 runs like a charm. 
 
I have never really had to pay particular attention to the setup of my system, since it has always worked fine for me. I have not needed to run my computer as a dedicated DAW. My next step will be re-downloading with anti-virus off (doesn't seem that safe to do, or a good idea, but I see people are suggesting that). And also installing with AV off. I will go from there.
 
Steven 

Yes, system specs please.  X3 has been the most rock solid release of Sonar IMO.  Sorry you are having problems, so let's see what you have for drivers, etc.

One thing to note when comparing X3 to X2, there have been significant changes to the code (at least it appears to me) to allow things like ARA and "better" implementation of take lanes, etc.  They re-wrote a lot of what is under the hood, so systems could react differently due to that.
 
Anyhow, many people have been down this path and a lot of times it just takes a few tweaks to get everything running well again.

madmaja
My next step will be re-downloading with anti-virus off (doesn't seem that safe to do, or a good idea, but I see people are suggesting that). And also installing with AV off. I will go from there.

 
It's not the downloading without AV that's crucial, it's installing with AV turned off that's essential. You can do installation without being connected to the internet.

FWIW, Avast was reported (several times) to interfere with downloading X3.

> X3 not only crashes itself but makes my entire system unstable.
> From there if I don't reboot my system other programs start to crash.
 
Something has caused Sonar and other applications to crash, it won't be Sonar doing this. Remember Sonar has terminated once it has crashed, it's not hanging around like a bad smell afterwards or causing applications to crash around it, nope.
 
This smells of something leaky, probably hardware/drivers, driver conflict, or the way ASIO is set up. These are just assumptions though we need far more detail.
 
Cheers...

scook
FWIW, Avast was reported (several times) to interfere with downloading X3.

Yup - I was one of those reporting that.  I had to suspend Avast during both download AND install.
 
Bob Bone