Slow shut down '90 300D

[2.5Turbo]

I was on a long road trip last weekend, and once when shutting down it took about 3 seconds for it to shut down. It did this every time from there on out. When I got back home, I whipped out the mityvac thinking vacuum leak. Sucked down the vacuum system and everything held 20" of vacuum nicely. Then I noticed that the vacuum line for the shutoff switch gets its vacuum from the brake booster line. The brakes work very well, but just to check I disconnected the brown shutoff line from the fitting and attached the mityvac. Everything was normal. I then hooked up the mityvac to the brown line with the car off and started applying vacuum through the ignition and into the IP. The stop lever would go down partially, and the vacuum would not build past 5 in/hg no matter how fast I pumped. I lightly helped the stop lever down further with my hand while pumping, and at the point where it starts taking effect on the engine fuel delivery it started to hold vacuum and went down all the way without my assistance. It held vacuum fairly well, definitely enough stop the engine. To eliminate the lines or switch, I hooked the mityvac directly to the shutoff device. Same results. I also tried it with the engine running. I shut off the ignition and of course the engine kept running, albeit with a bit lower RPM because the EDS was no longer affecting idle speed. I applied vac to the shutoff device and I got the same results. The engine speed would slow down gradually for about 3-5 seconds until it stopped. I don't have that much of a problem with it being like this, but today in a parking lot it almost didn't shut down on its own...so I'd like to get it fixed . I'm thinking a diaphragm in the shutoff device on the IP has split or something. Is that part that difficult to replace, or should I let the indy do it?