I own a 2003 Chevy Monte Carlo SS. It has 130,000 miles on it. It appears to lose power when trying to accelerate when the engine is warm. For example, cruising on the interstate at 70 mph for a short time, engine at normal operating temperature, when I need to accelerate to pass the transmission down shifts, rpm increase, yet speed does not. I think my catalytic converter is becoming plugged. It does not seem to effect acceleration when the engine is cold. Any suggestions?
Sounds like the catalytic converter to me too. If this happens at night, pull off the road to a safe stop and pop the hood. Look at the catalytic converter and see if it's glowing red. You may have to look under the car, and not under the hood to see it. I had an 86 Corvette that had the same problem and the headers and pipes before the converter would glow red due to the blockage. A technician can diagnose this problem too with a pressure check of the exhaust system before the converter. It will show high pressure before the converter if it's clogged.
A bad O2 sensor could be to blame, but you should be getting a check engine light too. Also a clogged fuel filter may be causing a lean condition at the high speeds you are talking about. I would have it changed if it has not been as well as crawling under the car to look for the red hot converter.
Listen to Kevin Bernie, he is right. I have the very same car as you 2003 Monte Carlo SS Limited Edition. It has 150000 Km on it. That's about 130000 miles. I could take off real fast when the engine was cold but not enough to spin the tires. This car could always spin the tires. But when if got hot and I tried to climb a double lane hill ( which I usually use the left lane because I am always passing all the Jap Crap that's polluting our North American Highways) I had to use the right lane because it would down shift, rpm's go up, car slows down. How embarassing! It turned out to be a clogged catalytic converter caused by long distance travel and cheap gas. It's an expensive fix. I think I will use better gas for now on.