Hi. My first post for 87 900 SPG. While driving at steady speed, not accelerating or slowing down the car stumbles. In lower gears it kind of feals like when you are first learning to drive a stick and you don't give it enough gas and the car starts bucking. As soon as I give it more gas it stops and takes off. It does this in all gears but doesn't buck in high gears just stumbles. Runs fine at idle. Any ideas on where I should start would be great.