1992 cutlass supreme 3.1 liter. i have 105000 miles and had no problems until i used a fuel system cleaner mainly methanol. now at low rpm's engine skips and bucks and is sluggish as speed builds up. once above 2500 rpm (guesstimate) bucking stops. accelarating to hiway speeds still seems to have less accelaration but no bucking. once at hiway speeds and engine settles into overdrive bucks and skips return. drop shifter to regular drive and the bucking stops. still sluggish.