sound volume drops randomly with background audio

I want to have music that plays for the first few slides of my project, then again at the end of the show. In between there are slides with narration (all object audio) which I do not want to have music underneath. I have buttons for users to advance from slide to slide so they can take time reading (or not).

I inserted looped background audio at the beginning, suppressed it on my narration slides with Stop background audio, then enabled it again at the end. So far so good

I then introduced a couple of introductory slides with narration, and set the background audio to reduce to 15% during any occurance of object audio. Great! Until testing and suddenly some narration audio on other slides also began to reduce level to 15% at random. Sometimes a particular clip would reduce. On navigating to a previous slide and returning, it would be normal. Then another that was normal before would, on replay or republish, get reduced. There does not seem to be any pattern to this.

I also tried slide audio distributed over the introductory slides, but my navigation buttons also stop the audio, of course. Putting separate object audio on each slide is choppy.

Has anyone else experienced this issue? Any ideas?

Captivate 9.0.2.437