I've been working with Rapture 64 bit, Dimension Pro 64 Bit and the Zeta 2.1. I'm able to record the parts without any hassles, I can then convert them to tracks no problem.
 
Further down the line when I'm doing some random editing, SONAR crashes. Luckily I can still open one of the projects but when I try to save that project again I get the dreaded "Unable to save file - Disk is full.
 
The only way I can get around this is to bounce to tracks as soon as I've finished recording the part and then delete the softsynth. Once deleted I save the project, close SONAR and then reopen and carry on working.
 
Not a great way to be productive. I can reproduce this crash virtually everytime. I went back to X2a and couldn't reproduce it there so it looks like something in X3c.
 
I must say that no other version of SONAR has ever been this unstable on my system.
 
I want to do some more investigation and then will send the info to Cakewalk.
 
Not sure how reliable that will be since I haven't has a single response to my query with support about a month ago but I'll send it anyway.
 
Cheers,
 
Peter.

what's your system specs? and can you give steps to reproduce, ideally from a new blank project?

Disk is full? How full are your hard drives? Check your hard disk read/write caches in Preferences (they may need to be increased). When editing, mixing or pretty much anything other than live tracking it's a good idea to increase your interface audio buffers because latency doesn't matter while doing those things and it will be less taxing on your system which means less crashes.
 
Just some things to look at. Cheers.

Oh and just to add to that... I had increased the hard disk cache in X2 and those settings did NOT transfer over to X3. They went back to the default. That may be a per project thing but I only noticed it after installing X3.
 
Another thing that might help pin down the problem is inserting the Performance module into the control bar and seeing what and when things are maxing out. Cheers.
 

Rapture has been unstable for years. I stick with Absynth 5 and Kontakt 5 now.

Hmmm...I've never had a Rapture crash or any instability, and I've been using it since the beginning. I use it on pretty much every song in some capacity. How does this instability manifest itself?

I keep hearing about the instability of Rapture too, but have never seen it.  Can't recall it ever crashing.  i must be lucky.  In any case, even f Rapture itself is unstable, that does not help the OP's problem since it seems to apply across other synths.  Can you take us through process step by step and do you have a project file you could share?

wasn't there an issue with the rapture installer at some point?

Check: http://www.cakewalk.com/Support/kb/reader.aspx/2003187
and check : http://www.cakewalk.com/Documentation/defaultJA.aspx?Doc=SONAR%20X3&Lang=EN&Req=Troubleshooting.16.html
 
(yes these are to fix different issues but please try them). 
 
Please run chkdsk on all your drives and defrag.
If the issue persists after this please go to the windows event viewer and post the error messages here, and post you drive space and how much is free.
 
Also we really need to know your system specs, could you update your footer to look like mine?
 
Cheers..

http://www.cakewalk.com/Support/kb/reader.aspx/2007013294
for the rapture installer patch, good luck