Engine Performance problem
1999 Other Mercedes Benz Models V8 All Wheel Drive Automatic 109000 miles

I have an ml 430. Its a '99 and has about 109000 mi. When I use it to run errands like go to the mall and different stores it will jus shut off.. the first time it was after errands and was parked in a driveway, i tried to turn it on after 5 min. of being off and it woulnt start. It didnt even make any noise like it was gonna start, so we jump started it and it worked fine, until 2 weeks later after another day of errands i was at a red light while entering the freeway, when it turned green, i gassed it and it wouldnt go, it just shut off and wouldnt turn back on. i had to get it towed to my house. When they took it off the tow truck, my dad got in and started it right up and it was fine! some people say its the alternator, some say its the fuel pump but I dont want to replace anything thats not needed. please help!