Dave, check two things, first, attached to the throttle linkage is a downshift cable that somehow actuates the transmission in relation to throttle pressure (your foot). This may need to be adjusted, easily done by loosening a threaded nut on the end of the linkage. Secondly, the downshift button located beneath the gas pedal may be bad and in need of replacing, its electrical I believe. I had a similar problem on my 89 420 and it was easily corrected. Good luck...