1997 Toyota Camry engine code P0420

I love reading your site and now it is my turn to ask you for advice. I have a 97 Toyota Camry 4 cyl with a code of P0420 Catalyst Efficiency Below Threshold (Bank 1). A couple of years ago I had this code come on and a dealer said I need to replace the exhaust manifold/convertor. I did this and they cleared the code. Later on it came back and I installed the first O2 sensor because the old one was re-installed. Now it comes on and goes out at no particular time. I know that the second O2 sensor was changed at a service station since then but I don’t know if it was factory brand, if that makes a difference in this case? Could it be as simple as a connection problem? Should I use contact cleaner on the O2 connections, erase the code and see if it comes back? The car runs excellent and no difference in mileage. I am stuck on what to check and/or replace. I look forward to your response and once again, you’re doing a great job with the column.