Has anyone else noticed this?

I have noticed on numerous occasions when I start out with an aggressive start, really hammering the accelerator, that it seems like the back end of the car moves a little to the right and then to the left. It almost seems like the car is going to fishtail, but in the end it doesn't. At first I thought about the tires, but upon inspection the tires look good and the tire pressure right in line with recommendations.

Any thoughts?

 

That's the traction control working. With so much power and especially torque, you are losing traction momentarily. What the traction control is doing is braking one or other of the rear wheels, as well as backing off the throttle, to curb any tendency for the back of the car to break away.

Without the traction control (and even if you switch it off, it's still there in background), it would be easy to execute lurid tail slides especially on marginal surfaces.

 

What Blue SL said and also since there is No LSD, the power is not bieng distributed equally. Merc needs to make a Real LSD on these cars, the TCS will not do.

 

Particularly with Renntech’s ECU modifications for power and throttle response, it’s hard to avoid that squirming feeling from the rear end of an SL600. I’ve done many silly things in cars, but one thing I’ve promised myself I’ll never do in this one is switch off the traction control. One moment of inattention, or an overenthusiastic twitch of my foot, and I could find myself driving sideways. With the traction control safely on, that squirming feeling is actually kind of fun …

I agree, a real limited slip differential would be good in this car.

 

That behavior is typical. You will notice it especially on snowy / ice covered roads -- even at lower acceleration speeds. The car's backend will want to go in one direction for a split second and then comes back in line. Was the same for the older R129 but more pronounced.