The condition you have described is not that unusual. Remember that the front wheels are correcting for the REAR wheel track & alignment. So if the rear is not tracking correctly & the front end hasn't been adjusted for that(4-Wheel alignment) then the car can "wander"!!
The shake you're talking about is "cowl shake" & SL's can exhibit this condition.
MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS M.B. TECH