Check out some of the posts on the subject of P0420 on [link|https://www.matthewsvolvosite.com/|Matthew's Volvo Site|new_window=true], which has been an invaluable resource for me and my Volvos.
The P0420 trouble code officially translates to "catalyst system efficiency is below threshold (Bank 1)", meaning there's not enough of a difference between the reading of the front oxygen sensor (before the catalyst) and the rear one after it. You've already done the obvious fix in replacing the catalytic converter, oxygen sensors and spark plugs. It's impossible to say if all the stuff they changed was 100% necessary, but given that it seems to have fixed the problem at least for a couple of years, it was the right call on their part to ensure the problem was fixed.
Some of the other causes described on the Volvo site include large vacuum leaks and exhaust leaks before the catalytic converter. I would personally start looking for any kind of exhaust leak starting around the exhaust manifold and working my way back to the cat. On one of my Volvos I found a cracked exhaust header, which could definitely cause the problem, but I had to remove the header to find it. Vacuum leaks could be another cause, but one bad enough to cause a P0420 error should also cause noticeable performance problems.
Another guy stated that his P0420 problems went away when he switched to premium gas, and he tested his theory by filling up with regular and sure enough the code came back. Guess it couldn't hurt to fill up with premium, clear the code and see if it comes back.
So hey, go ahead and replace the gas cap, fill 'er up with premium and see what happens!