I'm not 100% sure, but I don't believe his model has an idle speed controller like '92 and earlier 300E's.
His model has a throttle actuator, which has several functions, idle speed control being one of them.
Certainly does sound like a vacuum leak. I'd recheck all vacuum hoses to make sure they are attached and not cracked.
I'd also check the spark plug ends on the three coil wires. You could have a misfiring cylinder.
Remember that you have three coil wires and three spark plug wires, and they rarely go bad. The failure is usually in the boots or ends of the wires. They are about $7 each.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".