I agree with what's been said so far, but your technician must have a reason for suspecting the cat. You mention misfiring, is it setting misfire codes?
If the cat is breaking up and plugging the exhaust exit, it can do exactly what you are describing at low speeds and idle. This blockage can be intermittent. Does it ever cut-out at road speed?
It can take a few days to set a P0420 (cat efficiency) fault code as the PCM has to run and pass all the other O2 and EGR tests first before testing the cat. The cat monitor test passes last. The fuel tank has to be above a 1/4 tank to run the self-tests.
Changing the sensors with non-OEM parts can lead to the same failure again. This makes diagnosis more difficult as one does not suspect a new part as being bad and looks at something else as being the problem.
The early Mitsubishi distributors were a problem on the 2.5L and a used one is no guarantee that it is a good one. Even reman units can go through the reman process and go back into a car and still have problems. These can be intermittent which makes them even harder to catch or fault. It's nice when they just flat-out quit as that makes diagnosis and locating the trouble fairly easy. Make sure that the distributor connections are secure as I have found broken plastic retainers, missing clips and partially plugged-in connectors here.
A road test with a scan tool showing CMP and CKP sensors as being present while running is most useful as diagnosis. The moment that the car quits, see which sensor was lost or undefined first. Of course after the stall and the engine stops turning completely, both signals will be lost.
As the distributor (CMP) problem was cleared up, it seemed that the CKP sensor was the one that failed more often.
An out-of-sync issue can be caused by a skipped timing belt. The belt, idler bearing and water pump should be serviced at ~100k miles as a matter of maintenance. The distributor position is not adjustable.
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