STOP!
Before you buy ECU readers and whatnot, go to Auto Value and have them run the diagnostic test (free!). If there's no check engine light, chances are that they won't be able to tell you much, but it's better than paying a mechanic for a diag. The ECU reader is the same thing. You'll save the $50.
That failing, and assuming that you have some mechanical wherewithall, it sounds to me like either a TPS (throttle positioning sensor), exhaust (probably a plugged calatyic converter), or a fuel pump or filter problem.
Other possibilities include: O2 sensor, intake air sensor, or just plain old crappy gas. Stop filling up on the reserve.
The TPS will be the cheapest. Probably ~$100 and easily installed yourself (with a little help from a Haynes manual (~$20)).
The exhaust will be next cheapest at around ~$200 to take out the catalytic converter and pop in a resonator, assuming you don't have a second O2 sensor after the catalytic converter, in which case you'll have to replace the catalytic converter and those run anywhere from $300-$600 plus installation (no, you can't do this in the garage).
FUel pumps vary. sometimes they're inside the gas tank and that ups the cost of the part and the cost of installation. I won't even guess at how much it'll be for a Grand Prix, but I'll tell you that the part for my GMC Jimmy is $600 and the installation is 2.5 hours at a shop rate of $100/hour, for a total of $850 PLUS MOFUGGIN TAX.
I hope your problem is the TPS... }
