Check your battery voltage and charging system with a digital voltmeter. A few minutes after starting on a cold day you should read at least 14 volts at the battery terminals and no less than 13.5 to 13.8 after it's warmed up and running. Check it several times. Low voltage can cause a lot of unexplained problems.

Has anyone pulled an oxy sensor, stuck a gauge in the hole to read the back pressure of your exhaust system? Plugged catalytic or muffler, perhaps?

Local Auto Zone stores offer a free reading of any codes that might be set in your vehicle.

That's about all I can think of, offhand.