95 aurora poor acceleration / milage

I have a 95 Aurora, it starts great and idles o.k. Seems to have slightest miss at idle. I can rev it up and don't notice any miss or anything. When I take off it feels like its missing on half the cylinders but also feels like it wants to hit on all. I have noticed that the milage has dropped from 20 mpg to 6 mpg. I have changed plugs and wires. The dealer replaced fuel rail about 1000 miles ago. This problem seems to have started long time ago and got worse incrementally to the point it is at now. By this I mean it would run like a scalded cat with the occasional miss at high speeds( around 50-70 mph) while accelerating. But now just runs like crap except at idle.

Ps
I do have weak spark coming from one coil pack, someone suggested that be the first thing I replace, then go from there.

yup. change pack . checkout jegs.com for gm coil packs. also check wires. vitekperformance.com . denso iridium plugs. densoperformance.com

I had exactly the same problem. After spending nearly $1000 on stupid GM parts and labor I finally bought a set of NGK-Irridium spark plugs and changed 2 older coils from the coil back and also Borg-Warner spark-plug-wire set. Since then the car has been working like a clock and uses only 23-34 mpg average which is great for my 4700 pound car including me and my scooter and scooter lift in the back. In short: Coils+NGK Irridium plugs+Borg Warner-wires=problem solved.