Toyota - Sienna :: 1998 - Occasional Shudder Due To Fuel Changed?

I have a '98 Sienna that's still running well (271k miles) but about a year ago it started acting strange, just a rare but minor irregularity in the normally smooth engine. A few months ago I changed the fuel and air filters, but that didn't affect it. So I tried changing the tank pick-up filter and confirming that the tank didn't contain any gunk. Again, no change. Since then the engine light has come on, but it still runs well enough for now. My fuel pressure is solid (manual specs, 50psi I think). I'm not sure if it's too rich or too lean, but I'm convinced it's fuel related unless someone has a better idea.

My suspects: fuel pressure regulator (in the fuel tank), clogged fuel injector(s).

Other potential clues: 270,000 miles on the original engine. No smoke (that I've noticed). After changing the filters my fuel pressure was perfectly solid at manual recommendations (50psi if i remember correctly). New battery, new fuel cap (factory cap; sometimes store-bought ones don't vent correctly). Sometimes the fluctuation goes away for a day or two, while other times it's hard to accelerate uphill at highway speeds. The issue is not clearly affected by temperature (same when 40° or 80°). It was much less noticeable when I drove to Montana this winter; they have a winter blend, which might have masked the problem (or the higher altitude). It's possible I just didn't notice it as that drive killed my steering pump and a CV boot, so my focus was on other problems at the time. I found today that can I make it shudder a little at highway speeds: letting up the pedal for a moment and gently giving it a little more gas causes a gentle shudder, and very rarely what sounds like a small muffled backfire.