what would cause 2005 grand prix gtp to not accelerate over 30mph?

If the engine seems to be running fine, but won't go over 30, your ECU might be stuck in 'limp home' mode. This can happen quickly and for something as silly as trying to jump the car when it's already running. Try this: turn the key to the "on" position, watching the Service Engine Light (SEL) carefully. It should come on, blink off for a split second, then come back on and stay on. If you didn't see it blink off, then the ECM isn't working right. If the SEL stays on when the car is running, a code will be stored and you can find out the problem with any OBD (Pep Boys will scan it for free). If it goes off when you start the car, you'll need to dig a little deeper. With the engine running, tap gently on the ECM. If the engine runs rough for a second, the ECM is bad. Do the same test on the MAF sensor. I would also test your fuel pressure to make sure it's within manufacturer's specs. Last but not least, a bad ground can cause a lot of issues. Make sure the block ground is good and tight.