Nobody here will be able to offer much help without some additional info.
Do you know what codes specifically were stored in the control module?
Intermittent problems can be difficult to find and if the car runs perfect then all the technician can do is try to evaluate the codes that are stored in the system and make an educated guess or wait for the problem to replicate and then do hard component testing.
If your technician doesnt seem competent then you can always try a different one, or if you trust him maybe make arrangements to leave the car with him for a few days to try and replicate the problem.
Personally I wouldnt replace an expensive component (cat converter) on a guess I would want some concrete evidence that it is failing.
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