When I start my 03 3800 V6 (not supercharged) grand prix, the engine doesn't always rev up like it usually does. This typically happens when the engine is warm, but it's happened when cold too. The engine starts, but instead of revving up to around 2K RPM and slowly settling back down, it just goes straight to about 1K RPM. Sometimes it runs fine after doing this, and other times it kind of stumbles and almost stalls (but never has actually stalled). The engine runs fine otherwise, so I'm not sure what to think. Any ideas?