We moved to a house that has well water a couple of years ago - my first experience with that. It worked fine for most of our time here, but a couple of months ago we started noticing occasional drops in water pressure. Usually in the shower, though the kitchen sink flow is also noticeably difference sometimes. At first, we all just kind of ignored it, assuming it was because multiple people must have been showering at the same time. But over time it became more common, and we figured out it wasn't due to multiple simultaneous showers. And within the past week or two, occasionally the pressure drops enough that it's hard to shower some times. When the pressure drops a lot, it usually comes back after a couple of minutes, so we can wait it out.
We have a Well-X-Trol WX-302 pressure tank and a Grundfos pump (sign says model # 16505412 but can't find a reference to that online, so maybe it's wrong). The pump seems to have been installed in the mid 90s. The pressure tank has a pressure gauge next to it that seems to always read about 82 psi, regardless of the time of day and regardless of whether we're experiencing low pressure in the upstairs shower or not at the moment (I have a security camera pointing at it, so I can check it from my phone). It is so consistent that I'm getting a little suspicious of it.
What is the likely cause of this variable pressure?