intermittent starting problem

I have a 1998 ML320 with 62000 miles. I am having intermittent starting problem lately. The dealer replaced the fuel pump two months ago but it does not seem to be the problem. It normally happens after driving for a while. It always re-starts after sitting for a day. It died twice while driving (at low speed, low RPM) over the past weekend. The engine cranks but would not start. The engine stay cranking for about 4-5 second by itself even I have released the key. The key does returns back to the normal ON position after releasing the key.

The car is with the dealer this morning. I spoke to the service manager. My impression is that he is not going to do anything unless he can reproduce the problem. Any help is greatly appreciated.