99 Grand Am P0300 Random/Multiple Cylinder Misfire Detected

Hi I am having an issue with my 99 Grand Am. It has a 3.4L V6 3400 engine, with approx. 87k Miles. Here is information on what we've diagnosed so far

Engine Code:
P0300 Random/Multiple Cylinder Misfire Detected

- Changed the Spark Plugs, Wires, and Coil Pack with no result

- The car skips and backfires, and won't go much above 2000 RPM's then sounds like it's sucking down air.

- Won't go down the road above 10-15 MPH, and starts sputtering and skipping, esp uphill.

other known issues involved with the above code are:

EGR Valve
Catalytic Converter
Fuel Injectors
Exhaust Valves

If you know what may be causing my problem, have more questions, or have an answer, please respond, or email me @ schicherchia@aol. Com. You can also reach me on aim if I am not away, my sn is schicherchia. Thanks