What would cause a 2003 Monte Carlo to bog down under a load?

Since Oreillys said there was no problems I was trying to think of things that might be wrong so I changed out the throttle position sensor. Checked all plugs and wires. If there was a misfire then the scan would've showed it. The engine light was not on when I took the car in to be scanned. The light didn't come on until 4 days later. There are 214,000 miles on the car.

Depends, you can have an intermittent miss that is not long enough to set a hard code. Plus the scanners at Oreilly's are just simple scanners and do not read history codes. You can bring it to an exhaust shop and have the exhaust back pressure checked, like Greg's Auto said, to see if the catalytic converter is the problem or bring it to a shop with a more sophisticated scanner that can read current Misfire data and history codes.