Overvoltage protection relay can be bad too, this causes an intermittant loss of current to the idle control valve, and it closes too far. Worse if it is also dirty from leaking hoses.
Check vac lines and idle control valve hoses for leaks. Clean idle control valve and throttle mass air flow sensor venturi.
Have mechanic check the OVP relay -- if it is the original one with one fuse on top, it is likely bad, and will cause the ABS light to come on randomly, too.
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!