I've had it with this problem, (87 300E, 195K miles) I have an OVP relay on order, when it gets here, if it doesn't fix the problem, I'm about ready to take it to the dealer to diagnose what is causing the hesitation and rough idle! I've replaced:
Plugs, wires, cap, rotor, fuel injectors, fuel filter. Each item improved the performence a little, but the hesitation still exists.