Today I read the codes again, after having put more than 60 miles on the car since the last time, and now: No codes at all! So that narrows down what the potential problems could be. Thanks everyone for the suggestions.

400Eric's suggestion of a clogged cat is high on my list right now. They're original, and the car does seem to be exceptionally quiet. It would explain the lack of power as exhaust flow increases (or tries to increase).

This issue has been ongoing over several tanks of gas and several months, so I doubt it's a bad gas issue.

A clogged fuel filter is possible, as ps2cho suggested, although I replaced this (as well as cleaned out the filter in the tank) about 20k miles ago. I think the interval is 60k? Since it's cheap and easy I might just do it anyways.

I'll also clean the MAF sensor with some electrical contact stuff.

I'm going to take a look at a couple spark plugs as well. But I replaced these not so long ago.

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'85 500SEL, 245,000 miles (sold)
'92 400E, 295,000 miles (donated)
'84 VW GTI (work in progress)
'10 VW GTI (daily)
'72 Chevy C10