Unexpected huge db loss when mastering: need help

I'm experiencing a serious problem with a reoccurring issue that I have not been able to replicate at will.  When in the final stages of my projects and working with a full array of plugins, I have all of my signal routed to the master bus but the mastering plugins are on the preview bus (predecessor to the master bus).. while playing back the track inadvertently the signal drops by approx. 5-8db.  I notice this in the audio but also notice this in the waveform preview feature on the master bus.  This is very frustrating because as I'm in the final stages of mastering I'm making minor tweaks to instruments signal path output levels as well as mastering effects plugins.  Then suddenly, this db loss occurs and all of my hard work to fine tune the mix goes to hell. 
 
I am certain the loss wasn't due to anything that I did because I'm simply tweaking a few tracks at a time in small incremental increases/decreases as well as parameters on the compressors/limiters.  Furthermore, the loss appears to occur when I'm not even touching any settings.  In any event, any changes I make would account for such a drastic loss of db across the entire mix. 
 
The only way that I can correct this is by reworking the input/output of the final stage of plugins, but there again I run into a problem because, inadvertently the signal returns to "normal" and then everything starts peaking/red-lining.  Then I have to start all over to work on the levels I previously had.  The stress that this creates is incredible.  I then feel like I have to rush to finish the mix and render it before the problem occurs again.  I've tried, closing and reopening the project as well as restarting Sonar to see if the signal returns to normal.  The only thing I haven't tried yet was rebooting the computer.  But if that resolves the issue, it really isn't a practical solution.  I've also tried increasing the buffer size past 1024.  I've also tried globally turning off then back on all of the FX (bypass all audio effects).  But I've had no success.  It almost seems as though one of my plugins (a limiter or compressor) stops working, then comes back to life.  But none of the "engagement" lights stop running and in the effects bin, the color-code that shows that a plug in is ON still shows all of them on.

I am  very frustrated with this issue and wondering if this is a known issue.  Is anyone else experiencing this.  I believe I started noticing this approx. 2 upgrades ago but it wasn't so obvious to me because the projects I was working on where not at the mastering phase yet.  Is there a fix or workaround for this issue?
 
My PC and audio interface are working well within their processing parameters.  I'm running Windows 10 64bit with 32GB ram on a Xeon processor.  My audio card is the UA Apollo Quad + I have the UAD Octo PCIe accellarator.  So I don't think it's a resource issue.

It might help if you listed your mastering chain. Perhaps it's an issue somebody else has encountered with the same plugins.
 
The only time I've encountered such symptoms it's been due to a setting on the master bus: once I accidentally turned the Gain down, another time I'd inadvertently had Offset mode enabled.

bitflipper
It might help if you listed your mastering chain. Perhaps it's an issue somebody else has encountered with the same plugins.
 
The only time I've encountered such symptoms it's been due to a setting on the master bus: once I accidentally turned the Gain down, another time I'd inadvertently had Offset mode enabled.

I think I found the culprit.  It still doesn't explain why it happens.  I have 9 plug-ins on that bus, the L3 Maximizer from Waves that inadvertently changes its threshold.  I simply pulled the slider back into place.  The issue didn't appear to reset the plugin, it just moved the threshold.  I'll monitor this next time it happens to see if it's the same plug in that changes it's settings and pay close attention to whether or not it resets the plug in completely or simply moved the threshold.  I do not have any automations working on mastering plugins.


I would like to hear if anyone has experienced something similar though.

The only time I've ever experienced anything like that is when I forget to turn off automation write while playing back and I change something, either by mistake or on purpose. But what I didn't realize is that Sonar dutifully captured my changes.
 
Open the list of all automation that has been captured and look for something related to the L3...
 
Regards,
Dan
 

Sometimes I have inadvertently changed settings of something in a plugin when adjusting the level of one of the knobs on the GUI and then accidentally scrolling within range of another knob. The same thing can happen with click focus, where you are intending to raise or lower one of the parameters with click and drag and next thing you briefly snag another parameter nearby and change that too.
 
If you are making adjustments with playback going, it is often an immediate realisation as to what has gone awry. But if you have made an adjustment while sound is stopped, finding the offending level can be an exercise in the pulling of hair and the tearing out of said hair, from the scalp! (Especially if the affected level is on a tab(s) behind the main GUI display and might also be a parameter that you don't normally do much with.)
 
When A/B-ing different mastering plugs, I have quite regularly done the thing where I thought that I had deactivated one plug and activated the other, but managed to leave both of them activated. The results can either cause a volume explosion that hammers your limiter with some nasty distortion, but at other times it can do the opposite and drop the level. This can be either perceived where the levels are still reading hot but the sound has markedly thinned out. Or actual volume where one of the plugs just kind of dies in its output.

Thank you all for your input.  I'm going to pay a bit closer attention to how I move around my cursor and mouse wheel while playing back as well as confirming if the behavior is isolated to just the L3 or other plugins..  I liked the idea of automating the L3's threshold level so that it stays put. 
 
Thanks again.  I'll report back to this thread if I find that the problem persists.