I fixed a very similar problem on my mother's 88 300E by replacing every rubber vacuum line I could find, and replacing the rubber hoses from the fuel distributor to the idle valve and idle valve to intake manifold.
All the rubber parts were either rock hard or melting, and all appeared to leak. The hoses on the idle valve were very hard and loose -- this will affect idle mixture considerably, depending on whether they were leaking or not.
A tune up (plugs, wires, rotor, cap) and a long trip and it runs beautifully!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!