I'm working on a customer's 03 Isuzu rodeo with the 3.2 6VD1 another shop had put new catalytic converters and O2 sensors on trying to troubleshoot a bad hesitation problem. I've since fixed the hesitation (bad harness running to MAF was causing signal to drop out randomly and throw a MAF LOW VOLTAGE code) since I've fixed it every few days the truck will randomly throw a P0420 code. I've checked on scan tool, and everything appears to be working properly when I checked. That said the code is only thrown every few days with no apparent trigger. Does it sound like they replaced the cat with a defective one?
I'm going to guess and say it is not technically defective, but just smaller in size than the original. If you are clearing the code each time, it may take a few days for the ECM to pass all of the required tests before it even looks at catalyst efficiency. If the code is going away on its own and then coming back, then the efficiency of the new converter is probably just on the fence between pass and fail. This is going on the assumption that all of the o2 sensors are reacting quickly to induced rich and lean conditions.
A general rule of thumb when replacing catalytic converters is either go oem or larger if possible. Due to the price, many people opt to go with an aftermarket one and are soon haunted by the infamous p0420/p0430.