Volume drops 10db every 50 to 60 seconds

Cubase mixdown is dropping the volume by 10db for about half a second, every 50 to 59 seconds. Testing with other software (Waves, Audacity) rule out the Focusrite Scarlett. It only happens with Cubase. Started a couple of days ago. I have to deliver some final tracks YESTERDAY … so I need some help ASAP, thx. Above test is with three minutes of pink noise. Times are not sync’d with start of the file, so must be external. My PC is generally NOT connected to the internet, and no recent updates to operating system or virus protection .

Could it be that a plugin is in demo mode in the signal chain?

Yes, it turned out to be a plugin that I’ve used for 5 years, and for which I did an “update” 30 days before. Didn’t realize that I’d need to go to their website, log into my account, and download a new authorization code. If it was in the fine print, I missed it.