1987 E 300 Wont start (Need someone with some knowledge of Mercedes)

We showed up to change the starter of a Mercedes which was previously dead (the starter that is) and ended up finding a new problem. When we finished with the starter we tried to turn it on and it wouldn't budge the first couple of times but it finally started and it made us think all was ok till 5 mins later when it died. We tried to start it and it didn't work again. We checked for bad spark from the distibutor and the spark plugs themselves, The fuel pump and its location near the fuel rail, and we couldnt think of much after that. Need some help please. Thank very much