When I let off the gas above 20-30 mph it bucks
 

So been I bought my sc last month been doing all the normal tune ups spark plugs, wires, fuel filter/pump but when I drive for instance anything over 30 if I let off the gas it bucks or shoots down in rpm more than it should say I was at 2300 it would shoot down to 2k if I let off the gas fully and slows the car down more like driving stick any ideas??
thanks guys any help greatly appreciate

Hello,

It sounds normal for an automatic transmission, it has an ability to engine brake as well. If it brakes a lot faster than it should, then the source of your issues can be something other than engine or transmission, seized brakes or a bad wheel bearing could result in a similar problem. Check that all 4 wheels spin freely, the brake fluid looks clean, and all the calipers look solid with no tears in the seals.

Hope this helps and best of luck!