Im working on a 2000 Pontiac Gran Prix you can run it around town for about 30-45min and then it bogs down to about 10-15mph and won't let you go any faster but if you turn the car off and start it back up you can get anoter 10-15min of perferct driving out of it every time... ive changed crankshaft position sensor, all ignition coils, plugs wires, mass airflow sensor, catalyst(now its throwing codes for o2 sensors)and several fuel filters but now its really agravating... any suggestions

I thought you were on the right track with the converter (I don't know what could have happened to the sensors... )
You may need to perform an exhaust back-pressure test.
Also, a fuel pressure AND a fuel volume test.

I'd focus on the catalytic converter or other blockage in the exhaust system. It's likely the converter.

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Originally Posted by CaptainRich

I thought you were on the right track with the converter (I don't know what could have happened to the sensors....)
You may need to perform an exhaust back-pressure test.
Also, a fuel pressure AND a fuel volume test.

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So what do you think about the muffler do you think its clogged up

You said, "catalyst," and I said, "converter...."
Are you saying that in fact you replaced the catalytic converter?

You installed a NEW catalytic converter and are having the same symptoms PLUS you're setting codes?

You may have a clogged exhaust system, somewhere.
If there's no signs of external damage, you may need a new muffler.
This is where the back-pressure test would come in handy.

K thanks cause I did change the catalyic converter and still having these problems and throwing codes

I had a Mercury that did the exact thing a few years ago, it was the main computer. I replaced it and it worked perfectly. Don't take it to a dealer, they wanted $600 to do it and I had a friend do it for $125, that was 15 yrs ago so I assume you will pay more now.