There is a thing on most Chevrolet and GM cars that have what they call a, "Air intake sensor" or something like it. I had a camaro that use to do the same thing to me and it ended up needing the sensor for the air-intake monitor changed. I know it sounds weird, but after I changed it and it worked I never questioned weird things like that ever again.
The way I found it out was by going to my neighborhood Auto Zone and getting a manual based on how to find out what was wrong with the vehicle myself instead of paying a mechanic a lot of money to just hook it up to a machine. You get this manual that has codes that will flash up on your dash to let you know what the problem is and where it is. You just follow some easy steps and it will tell you. That simple!
Good luck!
Dee Dee, August 2005