[De-railer]

Could possibly be the catalytic converter. Check to see if the 2003 is covered under the extended warranty by GM. They increased it to 10 years or 120,000 miles. I just had to have my cat replaced in my 2002 3.8L w/ 105k miles....was clogged and made car run like junk. I was able to get it done for free because I went in armed with knowledge.