About the only thing you haven't checked is exhaust blockage. Catalytic converters (and mufflers) have been know to collapse and block the exhaust system, and the symptoms you report could be due to the high backpressure caused by something blocking the exhaust system.

Sticking a potato up the tailpipe of someone's car is one of the oldest practical jokes in the world. The engine barely runs and no one can figure out why.

Duke