Chevrolet Forum - Chevy Enthusiasts Forums - What is the problem?! Help!

I purchased a 2005 Chevy Cobalt from Bill Heard Chevy in Plant City, FL at the end of November 2007. Each month I have had to bring this car in due to a service engine soon light and a "check gas cap" warning. So far they have replaced the gas cap TWICE, found a large evap leak/loose connection at the gas tank, replaced the catalytic converter and the coil pack (car was running extremely rough and on the way to the dealership got onlt 9mpg!!). The last time was the evap leak - which was "fixed" June 5th. Yesterday the light came on ONCE AGAIN. What could be the problem?? This is driving me nuts..I bought this car so I would have something RELIABLE to drive..and it obviously isn't..The drive to Bill Heard is 45 minutes alone...I'm so frustrated! I did an "Auto Check" full report and everything seems to be fine - no accidents, no bad title (no water damage title, etc..) Any help would be appreciated! Thanks!