It was doing the same thing for me on my school iPad with .wav files (the most prominent one was Moon Patrol De-Rastered by Chris Allen, download at
https://bit.ly/MoonPatrolDeRastered for the file in question), and something that seemed to help (I can't tell if the issue is entirely gone or not, which is far better than before) was going into the equalizer and switching to "Flat", then back to "Off", and setting the preamp to 0dB. The fast fluctuations issue is entirely gone, the slow adjustment might still be there but it's not prominent. I didn't get a look at the default equalizer settings, but this procedure helped massively.