No adjustment. I think you're just experiencing the natural action of the tracks. With prop we tension, the track drapes over the drive sprocket and rests on the second set of road wheels. That slack has to be overcome when going forward, just like the real thing.

If you really want some realism on the way it drives (not recommended for battling), you can install a Turnigy chip in between the MFU and the Rx on the throttle. It sets servo speeds (in both directions) so you can delay the start and stop. It simulates inertia and is fantastic, but really hard to master the driving since the tank will buildup speed on starting and roll out a bit after you release the throttle. It's a real blast, but like I said, not good for battling since you'll over shoot everything or not accelerate as intended. It's complete customizable so you can add as much of the effect as you want.