most likely a vacume loss problem, also a check valve could be going bad. If you want to have fun trying to solve a puzzle, buy a vacume gage. about $20 at any auto parts store. locate your vacume lines under the hood and start testing to see if the lines are holding a vacume. the problem of loosing vacume after shut off, may be the storage tank for the vacume. The MB manual or CD has an entire section for trouble shooting on this subject. You should be able to isolate the problem, most of the parts are inexpensive. The DIY is time consuming.