Phil, what graphics card are you using? Do you have a different one you can swap out, and do you have the latest graphics card drivers? Many times in these forums when someone has a problem that a) doesn't exist in some other DAWs and b) people can't reproduce, a graphics card is the culprit (especially when it installs one of those "HD audio" drivers, but that produces other symptoms). I don't know the technical rationale but some have theorized SONAR handles graphics differently from other programs (maybe because it's not cross-platform?).

What Craig says is true.  My Nvidia drivers were messing with my UAD stuff.   I DOWNGRADED to my lesser,radeon on board video and the problems went away.   I don't play games so it don't matter....who knows why it's working.

I'm running UAD 8.4 in Win 10 and a have NVidia card (GT610). While I run current drivers I load only the graphics driver, which gets rid of lots of bloat. Not issues at all.

It's an Intel HD Graphics P4600/P4700. I think it's on the motherboard. But what's the relevance? Again, I have never ever seen this issue with Reaper, which I'm pretty sure rules out a graphics card connection and UAD. And note again that it is intermittent and is cured by saving/closing the project and re-opening it.  I'm willing to believe that there may be a Sonar UI option that has an effect, so maybe I'll turn off X Ray to see if it makes a difference, just as a random example of something to try, but otherwise I believe these symptoms mean that's Sonar. I could try a support call with UAD and see what they think.

See if they hem and haw! :D.  Also, there can be strange issues with the windows 10  firewire drivers.   You usually have three options with the firewire cards: 1. legacy 2. the chipset and 3. the generic microsoft.  Try all three (that is if your using firewire).  
 
 
Btw the irig37 bug is a real bug.  It keeps SPLAT from opening. I mentioned it in the problems section, don't know if they heard.  This has nothing to do with Phil's issue.

I have also had this issue with UAD2 pcie and Sonar. Across many different versions, OS installs, graphics cards and full system rebuilds. In my thinking it has to be a Sonar issue.

I still use my UAD pcie card with SONAR with no issues. I have only used it with X3 and Platinum, but have not encountered any problems on the pc in my signature.
Ps. It always helps to put your specs in your signature, if wanting help.
All the best.

I had this problem once. 1176 meter has been dead, but plugin worked fine. But I have not found any dependence and do not reproduce it.
Win10 x64, UAD-2 pci-e, Sonar Platinum 08 and lastest UAD software.

Updated my signature to show my system details. Good point. I hope it helps the issue which appears to be happening to a few.