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

Happens when accelerating from a stop or on inclines. Also has a hissing noise under the hood. Engine light just came on 12/28/2012. Had it scanned at Orielly's 12/20/2012 said there was no problems. Changed out the throttle position sensor.

by in Tishomingo, OK on December 30, 2012
2 answers 2 comments
ANSWER by on December 30, 2012
Why did you change the throttle position sensor? You could have a misfire on one of your cylinders or a semi plugged catalytic converter. How many miles on the car? Was your check engine light on when you brought the car to Oreilly's?
COMMENT by on December 31, 2012
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.
COMMENT by on December 31, 2012
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.
ANSWER by on December 30, 2012
POSS PLUGGED EXH DO TO BAD CATT. HAVE CK'D BY MUFFLER SHOP