Over the last week I've been doing a bunch of mixing of some demo recordings my band has recorded. I seem to have run into an issue that occurs fairly often when I make some edits – say, for example, I cut a note out of a bass part and then nudge it a hair to the left or right – and then I try to hear it, and there's an audible click, the master bus meter lights get all maxed, and nothing is audible. I can continue and do some things like save the project, or undo the move... the play cursor moves like it should, but I can't hear anything other than occasional random clicks. If I shut down SONAR and restart it, I can continue... but it's not long before it happens again. It's extremely difficult to mix this way! 
I have several plugins running on the vocal and guitar and bass tracks, including the RED 3 compressor and RED 2 EQ, the Softube TSAR-1R reverb, and Drawmer S73 Intelligent Master Processor is on the master bus. I suppose I might just be taxing the system too much. Some things I tried that seemed to help: deleting some tracks that weren't worth keeping; updating to the latest driver for my interface... 
Anyway, I know there are a fair number of users of my interface – Focusrite Scarlett 6i6 – and was wondering if anyone else has been encountering such issues. My system specs are in my signature... it's a fairly beefy system.
Thanks in advance for any suggestions you might make...

Have you checked the signal path to make sure you aren't maxing something out ?
Like clipping a plug ins input ?

I would suggest, instead of cutting out a note and nudging it, set the track Edit Filter to show transients and nudge that instead.

try enabling "auto crossfade". Also do a slight fade in/out on the end of the clip that is left exposed after the nudge.

Being an owner of a 6i6 that will not be the problem. It is a solid interface with stable drivers I'm using 1.8. 
 
Make sure you check the zero crossing thingy in preferences. 
You sound like your cutting and pasting during a "non" zero crossing so you will get a click if that happens. 
There are a few ways to avoid this,  one mentioned above. 
Another is to split it out leaving lots of head and tail but avoid the other notes,  and copy, and paste to a new lane where you can move it and adjust the crossing points. 
I also sometimes just copy the same note from somewhere else in the song and paste that overtop and adjust the overlap. 

Zero cross over has my vote.

how is your ripple edit set
sometimes i forget to turn it off

The other thing to try when this happens is toggling the "FX" button on the control bar. It could be a plugin or bus that gets pegged. I have seen something similar before and I tracked it down to some plugins in the pro-channel.

AlChuck
nothing is audible

 
I can also agree with the zero crossings as a better method of working but to suggest this would make sonar lose audio ? I don't think so.
I could see it making an audible clipping noise but nothing that would blow the roof off and lose audio.
 
It sounds more like a downstream FX, overloading because of clipping hard enough to drop the audio engine.

Wow, guys, thanks for all the replies! The zero crossing suggestion is off base, I'm not introducing clicks in the track by hacking off a piece mid-waveform, that would cause clicks for sure. What I described – maybe poorly, sorry for that – was this: I would hit Play, and SONAR would freeze, there's be a click or pop sound, the master's meters filled, and no sound came out, except for occasional random click if I left the file open for a time. That is, it didn't freeze completely, most UI operations seemed available to me, but playing the file resulted in no sound at all, with the master bus meter's remaining filled to the red.
What I did was to freeze a few tracks; then I could proceed with no problems. So, I suspect I was overloading poor SPLAT a little.