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Hesitation, P0420 and misfires

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I have a 2003 Legacy L, 2.5. When my parents owned the car, the head(s) were rebuilt & the catalytic converter was replaced (about two years ago now) due to them getting a CEL with P0420. They were warned at the time that other issues in the engine were contributing to the cat failing, but the other repairs were declined.


Fast forward to now; the CEL is on constantly, the P0420 is back and the car is exhibiting hesitation on acceleration and also throwing codes indicating multiple cylinder misfires.


Have replaced: the Fuel/Air ratio sensor, MAP sensor (no MAF on this year), Spark Plugs, plug wires, & ignition coil. I have also tested the Throttle Position Switch using the procedure found in the latest Chilton's guide and found it functioning normally.


Could a clogged exhaust system be backing gasses back up into the cylinders and causing the misfires, which, in turn, clog up the exhaust system further?

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