My 84' 500SEL exhibit a shakey idle after a run to the redline on 2nd gear.

This happened several times recently after I accelerated to redline on 2nd gear and then proceded to cruise for 10-12 miles in 4th gear between 70 to 80 mph (perfectly smooth running engine) and when I exited the freeway it would shake under 1500rpm. Above 1500 RPM it would smoothed out. The idle would return to normal smooth running after I parked the car for a couple of hours. Any thoughts? Thanks.