I've been working on this project for many months. It's over 15 minutes now and has a full orchestra's worth of EastWest instruments, plus 3-4 others, for a total of about 25 active tracks. I have some EQ and reverb (Sonitus) effects on the buses they are being routed to, about 6 buses in total. Everything's been going fine, but then a week or so ago I started to get a lot of clipping and stuttering, and this morning when I loaded the project (immediately after a system reboot), I was told I was low on memory and not all the EW instruments were going to load. I can't think of anything that's changed on the system, and like I said, the project has been running fine all this time. The only thing is I purchased a new inexpensive piano synth and tried using that for a while, but was unsatisfied with it and removed it from the project a few days ago. 
 
Windows 10
8 Gb RAM
i5 processor @ 3.1 GHz

have you tried "freeze" tracks? to gain some memory..
 
 
cheer martin

Does the problem persist after rebooting Windows?

Check you don't have stray effects on the east West mixer. Other than that it would be worth to look at what is using what during sonar with a memory monitor.

OK I rebooted Windows again, and this time got fewer issues. I always forget to freeze tracks, thanks for that suggestion. I also had a couple stray effects I'd been trying out that I disabled. Turns out all three replies helped a little bit :-). I'll monitor for a few days and see.

8GB of memory is bare minimum these days if you're running a fair amount of VIs, especially orchestral libs.  (Win 10 alone can use 3-4 GB of memory at idle).
 
It's possible you were previously running to the edge of the wall memory-wise, and a recent update has increased memory usage so that you're now hitting the wall.
 
You might want to look into bumping your ram to at least 16GB.

More RAM is good, no doubt. OTOH, increasing total RAM may not address the real problem, but merely defer it for awhile.
 
I have 16 GB, but have never actually exceeded 8 GB of actual memory usage, even with multiple sample libraries. Take, for example, the project I'm working on today: I have 3 instances of Kontakt (6 instruments total) plus Trilian, Superior Drummer, and Omnisphere. And two synths that are too lightweight to worry about. Total system RAM usage: 5.6 GB. That's total memory, not just SONAR's piece of it, which is only 2.8 GB. I do have some libraries that could eat up 2.8 GB all by themselves, but they're not in play today. When they are, I'm likely to freeze them as soon as I can.
 
My point is that 8 GB should be adequate for most people. The exception would be film scorers with multiple monster orchestral libraries, for whom even 16 GB might not suffice.
 
So if an 8 GB machine was unable to load a library due to insufficient RAM, and if it was able to load it previously, then I'd be looking for memory leaks. That's when a program repeatedly allocates memory and doesn't return it to the pool when it's done with it. Windows itself has been guilty of this in the past (back in Windows 95 days it was recommended to reboot daily to recover leaked memory), and although Microsoft has spent millions of man-hours addressing this it still happens sometimes. More often, it's a third-party component.
 
Open Task Manager and click on the "Memory" column, which will order processes by memory usage. See what's at the top of the list. It should be SONAR. If it isn't, something's wrong. If some of the names of the top 5 processes aren't familiar, google them. They may be processes you didn't know were running, don't need to have running, or ideally would not be running on a DAW.
 
Also watch memory usage over time. A process whose usage continually increases could be a leaker. Once your project has fully loaded (wait a couple minutes and then play the song start-to-finish to make sure), SONAR's memory usage should either remain steady or increase by only a few megabytes.
 
 

Bit, while that does sound reasonable, the statement from OP that got me thinking he had hit the wall was "has a full orchestra's worth of EastWest instruments, plus 3-4 others"...
 
I don't know any users who are doing full orchestra mockups with 8 GB ram.  I own East West Platinum, and even using the 16-bit samples, I don't know how far I'd get filling out an orchestra template before my 16GB of ram was used up.  Bump that to the 24-bit samples and I'd probably be stuck with only the woodwind and string sections.  And I have 16GB.  OP has 8GB and using EWQL Play for full orchestra... IMO that's a recipe for hitting the memory wall.
 
Am I absolutely sure?  Of course not.  Like you say, it could be a memory leak or something else.  I'm just passing along what screamed out at me as a EWQL user. 

OK, here's the update. Owing to bitflipper's comment about memory usage in Task Manager, I went there and found something called "System and compressed memory" that was using sometimes as much as 2 Gb of memory, and creeping up, when my project was open. I traced this down to a plug-in called Grand Rhapsody by Waves Audio, which I'd added a few weeks back. I removed those instances from the project, including all tracks associated with it, rebooted and re-loaded. Voila, the memory problem went away. 
 
One artifact remains which I don't know what to do about. In the plug-in manager, I looked through all the sections to make sure nothing from Waves reamined. In the DMO section there was still something called WavesReverb. I tried excluding it, and got an error:
 
"The plug-in WavesReverb has a registry entry pointing to blahblah...\dsdmo.dll but the file does not exist on your computer. The plug-in may have been improperly uninstalled or deleted from your hard drive. Do you wish to remove this plug-in from the registry?"
 
I click Yes, but nothing happens. Restarting Sonar makes no difference. This still worries me. How can I get rid of that and make sure there are no more Waves products available on my system at all?
 
@JonD: I have 23 EWQL instruments open in this project (and most of them are using both the standard mic and close mic samples), and Sonar is using something in the neighborhood of 2 Gb memory, until I play back, after which it hovers around 3-4Gb. Been doing this for years, never had a problem with it until this Waves plug-in thing. As of this moment, the whole project is open and plays back nicely. You might have something else going on if you can't do that.
 
Oh, and according to Task Manager, OneDrive is taking up half a Gb of memory, no idea why. That's a wholenother kettle of fish...
 

Kill OneDrive if you don't use it. It's an intrinsic part of Windows, so you can't actually uninstall it, but you can disable it.
 
As for your stubborn plugin that won't go away, open the Preferences screen and go down to File -> VST Settings. Under "Scan Options", click the Reset button and then re-scan.