2000 Bonneville Running Terrible

Hello everyone, I'm a newbie. I have a problem. SES is now flashing, the moment it started flashing, it started running terrible. On accelleration there is this awful shudder, like driving over a washboard. Also, feels like a tractor pull sled is being towed, no power at all. My gas mileage has been cut in half, costing me roughly $20/day for 70-80 mile trip. Took it to autozone, can't remember exact codes, but the guy said I had multiple cylinder misfiring, a bad EGR Valve, and both O2 sensors were bad. So, I begain by changing out the plug wires (but not the plugs...I know, I know), I have also replaced the EGR Valve. No change whatsoever in the performance. Engine is the 3800 w/ 130,000 miles. What should I do next?

Change the plugs and make sure they arent bosch.Obviously problem with o2 sensors and maybe your cat is problem which would explain your gas mileage and feel like pulling a tractor.Check o2 sensors and cat.

No matter what codes they pull, AZ always says you need o2 sensors. The CEL flashing indicates a misfire condition exists. Changing the plugs and wires would be the first step. I don't understand why you'd replace the wires but not the plugs!!! If you still have the printout they give you, post the codes here and we'll help you decipher them accurately. I would also suggest changing the fuel filter and PCV valve. Unlike the old days, the PCV is an important part of a tune up, as it feeds the MAP sensor it's vacuum signal. A clogged PCV on a Vin-K 3800 can cause driveability issues. Also, when replacing the PCV, don't forget to take the o-ring off the old PCV valve and transfer it to the new one or you'll develop a terrible vacuum leak and whistling noise.