1999 Buick Regal Tune Up

My Buick Regal has 117,000 miles on it and we doubt it ever had a tune up. While driving it last night it suddenly lost power and started acting as if it was going to stall. It got worse at stop lights. It never stalled however.
It is still running except it loses power when idling at stop lights. It sounds funny while driving. We initially thought it had a small hole at the resonator because we could see liquid dripping from it when gunning the engine. The muffler shop said we had some other problem not that.
I had it scoped and they said it was a tune up issue and was fine to drive as is.
He said because the economy is so bad he will give us a partial tune up and only replace the plugs or wires that are at fault. We have no money so we are game.
Will not running on all 6 cylinders cause the noise the muffler seems to be making and the sputtering. Also can that happen suddenly while driving. It did not happen slowly over time. I will say we have terrible gas mileage issues.
I got in an accident 5 days before this occured. It was a minor one.