The car idle rough while in drive. almost if its trying to cut off. but run great while driving on the road . also during a stop if you slightly press the accellerator it smoothes out . it now has 121,000 miles could it be time for a tune up? and would changing the plugs and wires make a differents?

Check your wheel tire type first

A tune up would greatly help. Plus change all filters especially FUEL filter. Another common issue w/Benzs is vac lines cracking, breaking etc... causing similar symptoms you listed. Most are buried in/around and under air filter, inspect all the vac lines closely. But again a good healthy tune up will give you a good "ground zero" point for reliability.