Ok. Here goes 95 se Was running a little rough at idle so I changed the plugs and wires. Got about 1/2 tank of Gas and drove 20 miles at 55mph I had loss of acceleration, feather the pedal and it would return So I'm thinking bad gas. I added cleaner & treatment worked well that night.Next day drove 5 miles and now it acts like its only firing on 2 cylinders????HELP!!!!!!!!!!!!!!!!!

Did you keep the plugs? what do they look like?

Might be a bad ICM or Coil pack. Could be a lot of things actually, IAC, Dirty Throttle plate, bad MAF, etc...

Is the name Bosch on the plugs or wires? If it is go get AC Delco plugs and a premium set of wires that are not made by Bosch. Bonnevilles simply do not like Bosch at all and have tons of problems with it.

Check the firing order if you removed all the plug wires at once and believe you got them all back in the right place. Seems like something happened during the tune up. Check the gap on the spark plugs and if they the right plugs for the car.

Look for something you may have unhooked and might have forgetten to put back together. The O2 sensor is right under plug 6, check if it got disturbed, broken or unplugged when you changed that plug.

What I really wanna do next is check the fuel pressure. I got her running today for a few minutes. If I kept feathering the throttle it would stay around 2500-3000 but as soon as I let up She would stall. Some light backfiring through the throttle body also. But like I said after the tune-up I thought I had some bad gas added treatment and She ran like a champ over the road all evening.But the next morning 5 miles down the road she cut out and hasn't been the same since..Plugs are (AC good ones $7bucks each)

Was it running OK before the tune up?
If so you could have a defective plug or wire.
It is possible to get one from the factory.

I don't buy fuel at all. The backfire through your TB indicates unburnt fuel. As in a lack of spark.

Verify your firing order/hookup. Facing the car, front three cylinders are 1 3 5 left to right. Rear are 2 4 6 left to right. Check your plug wires for resistance. Should be an even progression of increasing resistance across 135, then a little higher across 246.