2001 E430 4 Matic Some Help Pls!

Hello guys, I have a 2001 E430 4-Matic MB 41 k miles, I bought this car a couple of days ago. When I test drove the car while it was on "P" gear I felt a little vibrating on the car, and I told the dealer that this car vibrates, and that he had to check it out. He told me its just maybe the oil you have to change it or the car needs a tune-up. After 4 days, while I was waiting on a red light the car just went dead. I tried to start it again but no chance, after trying and trying after a long time I managed to start it again, and after 5 min into another light the car went dead again, this time I had to wait 2 hours in the middle of the night to get a towing company, no chance with that aswell I managed to start the car again and to take it home. I forgot to mention that when the car died the first time the "check engine" light came on. The battery seemed ok everything was working its just that the car wouldnt start. The next morning I went to check on the car and when I started the car started normally, after moving the key to the ignition a couple of times left and right the check engine light went off, but I noticed that the RPM while the car was on "P" gear it seemed very high, it started from 10 RMP than raisin up to 15 Rpm while the car was on "P". I dont know whats the matter with this car, it seemed in a very good condition when I got it and now I have this. Any idea what would cause this? Should I consider another car a take this car back to the dealer? Thanks a lot guys, I really appricieate your work, I hope I can get an asnwer soon, cuz I am very desperate.

All the best Mario :(