I have a 2004 Sprinter with 5 cylinder, had similar issue that kept occurring over a course of a few weeks, had a friend who is a good mechanic trouble shoot it with the help of info off of sprinter source forum and with help of Dr. A, who is a fantastic sprinter mechanice out of Pittsburg or someplace in Pa. working over the internet and phone. Turned out to be a bad fuse block which is under the steering column. Got one off internet for about $100.00 out of Atlanta and swapped out and it worked fine after that. Problem with fuse holder that powered the start relay. It is evidently a common problem.
Dave Spain
Cocoa Beach, Fla.