Start with the green and yellow vacuum line coming out of the firewall in the engine compartment. Check and see if this holds vacuum. If not, take out the pushbutton unit to get acess to the 5 switch-over valves. I just described how to test them in a post about an hour ago. After checking the switchover valves, pull the vacuum hoses from the lower port (the one pointing towards you) of each switch-over valve, and see if they hold vacuum. Each of these lines goes to a vacuum element. If you find a line that leaks, be sure to test the vacuum element directly before spending any money on it because you may just have a leaky connector. The important thing is to find the leak and plug it up. If you have a leak anywhere in the system, it tends to make things behave strangely.

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1982 300D, black w/ palomino leather, 203K
2001 E320, champagne, 105K (Mom's)
1999 Dodge Dakota, squeaky & battered, 142K