If you block off all hoses (including the brake booster) and you vacuum pump is still producing 17.5 in, it is low. If my memory is correct, my 300SD gave a reading of at least 22 inches.
After you use the brake few times pulling into the garage, you use up the vaccuum. The pump is not able to supply enough vaccuum for the brake (due to slow idle plus a defective pump or vaccuum leak) and for the engine shut off. A work around for the shutoff problem is to let the engine fast idle for seconds (to build up vaccuum) before shut off. Now you have to test the pump and the hoses to see where is the problem. My bet is on the vaccuum pump. I think your pump is the diaphragm type. The diaphragm can break or the valves go bad.