1992 245 210k automatic

The saga continues...and I wonder if all of this is somehow related. To recap: my brick stumbles, sputters or bucks intermittently and totally randomly, going on 3 months now. Fixed/replaced: 3 vacuum leaks, fuel pressure regulator, crank sensor, ignition amplifier, fuel pump relay, flame trap, AMM, ECU, cleaned/adjusted throttle body, checked/cleaned ALL connections, wires, fuel pressure is fine (whenever checked), oxygen sensor new, spark plugs clean.

No OBD codes.

Some sudden oil loss 2 weeks ago w/o leaks, but hasn't leaked since? Compression 130-135-130-135. Some piston slap. Inside of tailpipe is sooty, but no smoke visible to me, although a friend of mine followed me and said he saw some black smoke under load. Mpg 20 city 25-27 hwy.

Yesterday it stumbled/bucked more frequently, all speeds. There is also a loss of power uphill or at hwy speeds, I press the gas pedal and it won't accelerate at all - but again, only intermittently. And the new symptom now is that the transmission doesn't seem to upshift from 2nd to 3rd to overdrive the way it used to. Sometimes it stays in 2nd or 3rd too long. I don't have a tach but from listening, the rpm go higher than normal before shifting, but not terribly so. Just a feeling that now it should shift but it won't until maybe 5-10 seconds later. And overall the transmission seems to act a little off. Fluid was changed 2 years ago, time for another change I guess, but looks good. No noises from tranny. I never tow anything and no mountain driving.

So I wonder...could that be a clogged - or intermittently clogged - cat?

What are your recommendations for where to get it checked/replaced? I could take it to the independent muffler shop, they replaced a cat before and that went well, but they are kinda hit and miss.
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1992 245 210k