When I first picked up my '92 300D 2.5turbo it had the same problem a few times. It did indeed have vacuum issues (the tranny does rely on a vacuum pump, which in my case had failed), so we replaced the pump. No problems since

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo