1988 Mercedes 300 E 2 yrs car stalling

[fantasia]

For almost 2 years now my Mercedes has been having a problem stalling. 1st my ABS light on then off intermittingly and now the ABS light on all the time. 1 1/2 yrs ago the car started this problem & finally figured out if I tapped this little black box under the hood on the front left then it would start, after stalling at a light or behind a car. Now it stalls when I am driving down the road with no warning, at 1st when it was going to stall it would seem like it was losing power, now it just dies. Anyone have an idea of what is happening? I really don't have the $$ to go to the dealer MB, so I have been driving on a prayer lately. Has this happened to anyone before?
Thanks (please help)