vacuum leaks at the modulator on the tranny is where I would first start. It could be something internal which means its cost you some money. It also could the modulator on the tranny. to get at it you are gonna have to remove the tranny. I haved a removed a tranny from a 91 190E 2.6 and the hardest part is getting it out, installing isnt so hard. You will need many extensions and a breaker bar to losen them up.