Ever since upgrading to x3c Many of my old projects would not work - some tracks would not play at all, sometimes the project would crash within seconds of loading, etc.
 
I tried everything suggested on this forum (plugin reset, etc) - Even did a full clean install of Windows 8 to eliminate any chance I had  tweaked something in my sleep:-) (I don't think I had messed with anything, but I wanted to be totally sure).
 
After a clean install of everything - the same problems were there!!
 
So I started loading in safe mode and picking one plugging at a time to exclude (lots of fun).  What I discovered was GlissEQ by voxengo (64bit, latest version) was the culprit.  In my older projects (before using the prochannel eq) I used this eq extensively - so all my old projects were messed up when I updated to x3c.
 
The weird thing is that often times the tracks that wouldn't play didn't have Gliss even on them, but just having Gliss in the project seemed to mess it up.
 
I tried putting many instances of Gliss in the demo song (on the tracks, on the busses, on the clips, etc.) and could replicate this behavior.
 
Could anyone with GlissEQ 64bit try to replicate this??

The biggest problems I've had with DAW stability have related to plug-ins.
 
I was at a PreSonus user's conference and someone in the audience who was running 64-bit Studio One Pro (which doesn't run 32-bit plug-ins natively) asked what the solution was for running 32-bit plug-ins. Wolfgang Kundrus, the developer, said "Tell the manufacturer to make a 64-bit version of the plug-in."

Don't get me started on plugins :)
I hope the vst3 spec will allow it to be better at discarding faulty plugins. One day I would like to see some sort of burn in plugin diagnonsis app.

"to eliminate any chance I had  tweaked something in my sleep:-)"
 
sorry but that's flippin hilarious. i wonder if there will every be a release of windows that doesn't require good karma and mad voodoo skills to run well?????

CakeAlexS
Don't get me started on plugins :)
I hope the vst3 spec will allow it to be better at discarding faulty plugins. One day I would like to see some sort of burn in plugin diagnonsis app.

Christian Budde has some VST testing tools, though i don't think any are of the "burn in" type...
here be linky: http://www.savioursofsoul.de/Christian/programs/tools/#VUnit

Just a little update - Getting rid of the Voxengo plugins really make x3 work extremely well.  Many of the little peculiarities I have noticed in sonar since x1 days are now gone.  
 
On another note, The included plugins in Sonar x3 are extremely good.  After going through a couple old projects and subbing out plugins for ones included in Producer, my mixes sound better (to me at least) and it was easier to get them to where I was satisfied.
 
Now if we could just get that gapless audio engine:-)