I am on X1D and still have a number of problems: some which have been here a long time, and some which have appeared recently.
 
Examples:
1. Snap is messed up.  When I draw notes or try to drag select, the times do not snap to even quarters, bars, etc.  I get odd values like 4:04:37 or 2:1:109, etc.  This was documented in a previous thread and all the common and obvious solutions were tested.
2. I lose audio if "too many" VIs are installed.  "Too many" can be 7, or 11, or 12 or some other number.  Task manager says I have plenty of memory and CPU available.
3. Copy/Cut/Paste sometimes work from short-cut keys (Ctrl+v,c,x) and sometimes don't.  I sometimes get strange behaviors from the corresponding menu options.
 
Several people suggested upgrading.  That's fine, but I'd like to know why I have issues other users don't have with a stable release.  Suppose I take the money and time to upgrade and still have these problems?  Without understanding what's going on, an upgrade is like kicking the TV to fix the picture.

There is a demo to try if you are interested. X1 was a version at its beginning. It did have bugs. Since then we have had X2 and X3 and now Sonar 2015. We looking at Sonar 2016 coming out soon. I can tell you CW has done an outstanding job of fixing the bugs.
 
I think those that have been advising upgrading are giving you very good advice.   

X1 was known for bugs, being an early Skylight release. Most people couldn't wait to move to X2.
But if moving to Platinum helps with your specific issues is hard to say.
New features get added, only to be found buggy. Latest example is Vocal Sync.
 
Worrying is when you say "some which have appeared recently".
I assume this is on the same X1D, same system, features you have used a long time?
 
But I sense a sincere push from the bakers to get the accumulated technical debt paid off.
Even Noel gets involved. This gives me heart and I am taking the time to diligently post repeatable bugs.
 
So for what it's worth, my advice would be to wait a bit.

konradh
Without understanding what's going on, an upgrade is like kicking the TV to fix the picture.

I hear you. I've geeked with computers and software for many years, gradually becoming the go-to guy among all my friends and relatives when something digital needs fixing. I started off wanting to diagnose exactly what went wrong, but long ago I decided that it was more important to get back to work than to know what the problem actually was. Most of the time it doesn't do any good to know, anyway. So kicking the TV is as good a technique as any in my book, if it works. An upgrade to new Sonar might fix things for you, or, as you hint, it might not. You can accomplish roughly the same thing for free by uninstalling/reinstalling the version you have, and maybe using some kind of registry cleaner to tidy things up there.
 
Other than that, I got nothin'.

as stated, X1 was the first release of a totally revamped program...so they could more easily update and upgrade.  Consider you're about 4 versions back at this point, in which although new bugs have been introduced, the program IN GENERAL has become more stable as releases come out.  I personally think the current version offers an incredible amount more than the first X series.  Also as stated, try the demo....but you're not kicking a TV to fix it.  You're buying a new 4k flat panel and ditching the old tube with the VCR built in.  Yeah..I don't think the new flat panels have VCR's built in.  Not being sarcastic, honestly....but you outta upgrade if money isn't a stopping point.  There's monthly membership payments too.

Update to Platinum! Stable as a rock on my system. The best version of Sonar yet! Worth every penny! I sound like a Cakewalk commercial. Yikes.

There were so many negative posts here when x1 and x2 came out so I stayed with 8.5 until X3. Things seems to calm down and everyone was pretty happy by then. As it is I will always recommend the upgrade. It cost very little when there is a sale. Make sure your signed up for the newsletters and e mails. 
There's not much point complaining about an old version as 98% ( wild guess)  of us have moved on. That's how software works. Upgrades are for making improvements and old versions become unsupported the next day. And X1 was even under Roland which is long gone now. 
Try complaining to Micro soft about XP... Lots of products have good and bad versions and years, to bad you got stuck with the lemon...
8.5 was good, X3 was very good and all of the new releases have been solid for me. 
 

Also even though your concern is stability which is superb in Sonar Platinum. Once you have a look at things like Mix Recall or Patch points or the Drum Replacer I believe strongly you will sound like us in very short order. The list of improvements and new features is lengthy and impressive. Plus there will be no new versions as such. Just constant improvements and features added. That maybe difficult to get ones head around but its what this year is all about. And next year is going to be the same. Its time to upgrade. 

Without reading this thread, through this weekend I've just been marvelling at how great everything is now.
 
Sonar is fast, fast.  I don't have glitches occurring.  I'm doing more complicated and better stuff than ever.  The new patch points routing is a joy.
 
It's just great. 

+1 Do the Demo.
What can you loose?