inability to accelerate in gear or get beyond about 1800 to 2400 RPM in park/neurtral on 2003 Pontiac Grand Am

I have about 31K on the car, it has the auto trans. This started about 10 miles after a fill up and has continued and gotten worse on another tank. I have checked fuel pressure it is 53PSI, changed and replaced the MAP, throttle position sensor, the coil pack and put new sparkplugs in. The OBD has shown two codes one for intermittent misfire and another for cylinder 4 misfire. The problem is so bad I cannot drive the car. Also the traction control shuts down and the check engine light stays on or blinks. Help! I got this on the GM family plan as a brother-in-law is retired from GM, should have bought the Mini Cooper

by in Fredericksburg, VA on April 10, 2009
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1 answer 1 comment
ANSWER by , April 10, 2009
Did you replace the plug wires yet? That is common. Ok, when you said coil pack, that usually implies wires. If there are just boots, then there is a coil, per cylinder. Is this correct. I would next do an exhaust backpressure test. If you had a misfire for a long time, it may have damaged/plugged/restricted your CAT Converter.
COMMENT by , April 11, 2009
There are no sparkplug wires but I have tried changing the boots with the spring inside which connect the coil pack to the individual sparkplugs.