My 1988 740 turbo has recently started to experience an idle problem. The idle will drop to around 500 rpm (engine hot or cold) and then the car stalls. I have looked around for vacuum leaks and can't find one under the hood. However, I am hearing a hissing sound coming from behind the heater/defroster control on the dash. Could this be the source of my trouble? I was thinking of disconnecting the vacuum hose that runs from the top of the valve cover to the passenger compartment (driver side) to test my theory. Is this the correct hose? If this is my problem how difficult would it be to fix properly?

Your thoughts are greatly appreciated.