1987 300E Cruise Control not working

Cruise control isn't working, and was wondering if anyone knew whether it was mostly likely the cruise control servo, or the computer "brain" that was mostly likely the problem. Also, is there anyway to trouble-shoot the system to see what the problem is?

Have checked the fuses/breakers, etc.

UPDATE: Had to do some dash work, so pulled the instrument cluster out, which also required unhooking the vacuum line that goes to the "economy" gauge. It appeared to be properly snug when I took it off. Re-installed everything, including the vacuum line, and now this morning the cruise control works, to a point. Still may have some vacuum leaking somewhere, causing the cruise control to "kick" out sporadically. Will have to notice if this is happening under increased load (more vacuum required other places).