Can not figure out why my truck is back firing. It only back fires when you go down big hills when driving on the road. As soon as you get to the top of the hill and start going down if you don't either kept the gas on just barely or the brake barely on all the way down the hill then it will start sputtering from the exhaust before the loud back fire. We thought it was bad gas and put treatments in it. We also changed the plugs, distributor cap, rotary button and we still are getting the sputtering and back fires. We only have the one vehicle and can't put it in the shop to find out so can you help with some recommendations so we could fix it on our own.