Digital servos really want to be exaxtly centered. Like the above guy said they have more steps in the movement. Like frequency in a speed control the more steps the more resolution and precision throughout the travel. There may be a little binding in your steering or throttle, but that might not be your problem. I think it's just time to put some new gears in your servo, they develop slop and the distance between the teeth on the gears is more than the steps in resolution on the servo, so it hunts to center moving back and forth really fast. Especially when the wheels are off the ground, like on startup on the box or on the gate or while refueling. Really it's not going to hurt anything and won't affect your driving, but if you want it gone, and it is a little irritating I know, you just need to order a gear set for the servo. It's easy to replace the gears, just be gentle. And you could have some binding in the servo saver. The two top screws that hold the top plate to the two posts, or some tie rod ends are gritty or, and it happens alot, the pivot balls are too tight, like in a mugen. As for you the throttle servo is twitchy and it's a new servo. You may have too much strain on it at neutral. The brakes should be free at neutral, so try loosening the end stops on the brake rods f and rear. And lessen the end points on the brake side, and make sure at full throttle the carb isn't all the way slammed open, give it a little room forchassis flex and so on. about 1.5mm and when at neutal make sure the rod that opens the carb isn't tight against the stop on the end when you give it gas it needs a little gap there too. 1mm is fine. the part that the servo arm pulls to open the carb with the set screw. And finally check the brake cam that pushes the brake pads against the disks. Sometimes the bottom that fits into the diff holder is tight and can cause weird issues too. Just make sure everything is free moving and not crammed together. Check all the wires and make sure there aren't any damaged wires going to the reciever. And what radio are you using, my old fm radio did that, but if it's a 2.4 then that's not it. Try a different servo, if it goes away then send in the servo to be checked out.....Stuff happens....But I thing somethings jammed up like the above guy said.