So, I'm using my octacapture and monitors to just play back Spotify music on my computer.  I get a lot of fading in and out in my right monitor. I've ensured that my Octa-Capture drivers are up to date.  Anyone else have this issue?  If so, were you able to find a solution?

I used to have an Octa-Capture.  Does this only happen with Spotify?  If you play an audio file already on the PC or play youtube does that also fade out?
 
There is only one driver for the Octa-Capture in Win 10, the plug and play driver. 
 
You also list the M-Audio FTU (which I used to have, too) in your sig.  Have you tried using that unit? 
 
 
 

I have Octacapture too and have not had this issue.  Spotify has settings for fade in and out (I think).  You may want to check those

First step is to confirm (or rule out) that it's Spotify causing the problem.  Try some other audio source and see what happens.
 
 

Hey all.
I've noticed this same issue when streaming music from YouTube as well. 
I have not tried to plug back in my other audio interface. It's buried somewhere. Not even sure where it's at anymore.

I had a similar issue with my m Audio device and it turned out to be the latency setting was too low.   I had adjusted this when a pal was trying his keyboard and noticed the latency wasnt great. When i adjusted this it screwed up everything and i got this waving in and out of the sound.   Wonder if you might be having a similar issue? 

I've had an Octa-Capture since they came out and i still love it.  
 
i had the same problem but discovered it was my audio cables.  I went out and bought new and good audio cables (Don't by Cheap cables) and replaced them as well as other cables connecting my instruments and Mics to my Octa-capture and WOW the sound was better.
 
It may be the AUDIO CABLES !!,  they wear out too.