you can test the stiffness of the spring, press the tip of the damper bolt against a hard surface. When the spring gets weak the damper bolt loses the proper "push" against the rack.
At least 2 other factors may be involved with a loping idle; a tired engine shock can aggravate this condition and the condition (balance) of the injectors affect the tendency for the engine to lope.

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body. '87 300D w/ Sportline suspension. No more '79 300TD w/ 617.952 engine at 367,750, it was my annual driver to Texas and I sure don't miss it poor A/C heater system!