2001 Pontiac Bonneville intermittent miss

My 2001 Bonneville has 422,000 miles and runs great, but recently I noticed it misses when climbing hills and seems to have lost some power as well. At 1st I was thinking it was an electrical problem such as a plug, wire, coil, etc, Now I'm wondering if the problem could be the catalytic converter. I recently replaced it with a used one that had 220,000 miles on it and it ran great after that, however recently my engine got very hot climbing a 16% grade in 100 degree heat. The coolant did not boil out, but the car was up to 270 degrees when I shut it off to cool it down. It was 2-3 days later I noticed that it started missing. It still goes 107 mph before the govenor shuts the fuel off, but it takes a LONG time to get there. Where should I start to look for problems? Thanks. Notapromike