When I set the volume low, to fall asleep, a minute later, mid-song, the volume pops up several levels. When I put the volume back down, it just pops up again a few minutes later. If I give up and just leave it alone, several minutes later, the volume pops up even louder. This has nothing to do with intra or inter song volume. Given the number of times I lower the volume, there should be no volume at all. Even setting the volume to one click above inaudible, does not help.
One thing to note, we don't alter music equalization on Pandora from how it appears on the original releases, unlike terrestrial radio, which compresses the entire broadcast. As a result, some songs sound louder than others. However, we do use the Replay Gain technique that encodes a fixed volume adjustment into the mp3/mp4 header so that the actual player (web browser, phone or device) can adjust volume just as the track begins playing. If there are vast differences, it could also be due to different instrumentation or arrangements causing different apparent levels of volume where the actual decibel levels are the same.
However, it sounds like this issue just starting happening during your listening sessions.
Let's try running through these troubleshooting steps to see if it helps with the issue you're experiencing: Android Troubleshooting