First, check all vacuum hoses for deterioration and hardening, replace any that are cracked or hard. A good way to check for vacuum leaks is to spray carb cleaner or starting fluid at suspect areas with the engine idling. When you spray in a certain spot and the idle goes up, you've found your vacuum leak.
Or you can use propane. Adapt a long piece of vacuum hose that fits tight onto the head of a propane bottle. Propane is safer to use than spray flammables, but can still "flash", so be careful.
95 E320 Cabriolet, 131K