A friend of mine had a similar issue some time ago and it was all related to some programs he installed on the machine.

Have you done anthing to your PC that would have affected the registies?

Z3ta+ is hardly a CPU intensive plug in so I doubt it's anthig to do with buffer size.   Especially if you already had a blue screen.

One thing you may want to try is to uninstall your audio drivers, reboot, install them again and the let Sonar run its performance test.   See what happens after that.

and a daft question regarding your interface driver... I assume it is the latest one downloaded from the manufacturers website, and not a third party ASIO driver?

It must have the latest drivers because there are no new drivers on the website.
I also tried uninstalling and reinstalling the drivers, still no luck.
Sorry for not knowing this, but what changes would affect the registries?

Which version of Sonar are you running?

It is X1 Producer Expanded

1) ASIO or WDM? Edit - Sorry you answered this - it is ASIO - have you tried WDM?
2) USB 2.0 or 1.1 (the port on your PC you are plugged in to) (or 3)? Also are you plugged directly in or into a hub (if Hub, is it powered or unpowered)?
3) What bit rate and frequency of your recorded tracks?
4) Other USB devices - 

oh and
5) What is your DPC Latency?
http://www.thesycon.de/deu/latency_check.shtml

Windows does all sorts of things even when you think you're not using it that much.
Registries get affected for the most when you:

- install/uninstall programs
- install/uninstall hardware
- run updates
- move folders locations (especially important for VST libraris)
- run supposed "pc tune up" applications
- setup windows to self optimise (or destroy )
 
If you say that you didn't used to have this problem but now you do, maybe something happened there.  The reason why I go down this route is because of your mentioned blue screen.   If it was a Sonar/Audio/Latency issue, at worst Sonar would have crashed.  But a blue screen???  That screams drivers/registry to me.
 
Do you get the crackling sounds now on other projects too or only the song you did the other night?

@Brando
WDM won't work with the E-MU as its drivers don't support it
It is USB 2.0 and it is plugged directly in
I don't have other USB devices as far as I know..
Latencies are:  
Current Latency around 120
Absolute Maximum is 448
And sorry I don't know how to check frequency or bit rate..


@massimo_max
I haven't done any of those things lately 
Well at the moment I only get the crackling on my first project, but if I were to keep on adding more tracks to my other project, it would eventually get crackly (I tried this.)

....