My bet is on a plugged/partially clogged converter. The test procedure is pretty simple in theory, but pita in practice. A tool exists for checking the backpressure via the o2 sensor port to see how much pressure it takes to go through the exhaust. something that runs progressivly worse when warming up screams a bad converter to me.

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95 Honda Shadow ACE 1100.
1999 Plymouth Neon Expresso. 2.4 swap, 10.5 to 1 comp, big cams. Autocross time attack vehicle!
2012 Escape, 'hunter" (5 sp 4cyl)