Make mine another vote for an aftermarket converter…if it turns out to be that and not the oxygen sensor (see Mike’s post…and know that the signal from the sensor can be looked at easily on a scope to be sure).
All converters have the substrates coated with platinum-palladium, a rare mmetal in the rhodium family. Rhodium itself works a bit better but is just plain cost-prohibitive.
My experience is that the cheap units last indefinitely.
Be sure you get a “direct fit” aftermarket replacement. It prevents the butchering that often is the result of a “universal” replacement.