Random volume changes playing music with iOS 8.4

Since upgrading to iOS 8.4 I get random volume changes while playing music. Sometimes mid-song the volume will jump either up or down. Often it fixes itself after a few seconds; other times I fiddle with the volume buttons, stop/start playback, push the sleep button, etc and it comes right. I haven't specifically tried fast forwarding like many people. I'm also trying to confirm that the number of bars in the volume "box" changes when these jumps happen - thus far I think it doesn't reflect the volume changes.

I've seen a few similar posts like below, though they are not quite the same as my porblem:

problems with volume decreases even with the highest volume

Volume issues with update to iOS 8.4

This has only started after moving to iOS 8.4. However, a related problem is that music volume has always decreased after watching a Facebook video. FB app needs to be close to return volume to normal (wish Apple would fix this too).

I use MediaMonkey on my PC for my music collection. This adds a "volume check" level to tracks when added to the library, the same as "Sound Check" on iTunes. This has never been a problem before though.