Throttle hard to depress and max RPM = 2000

Hello all. New to the forum and have my 1st Mercedes.  A 1990 560 SEC.  Problem - throttle is hard to press down and RPMs only go to about 2000.  Top speed is 20 MPH.  If go under hood and manual move linkage can get RPMs up to approx. 3500 but link still hard to move.  Any suggestion as to where to start?  All linkage is in place, no bends and no obvious restrictions.
 
Thanks   Bob

Have a 1990 560 SEC and need to remove the door panel.  Have the maintenance CD which provides some basic instructions, but looking for any tips from someone that has removed their panel.  Note:  reason need to remove panel is window tries to go down when press button in center console but window appears to be jamming.
 
Thanks   Bob