I'm no Mercedes Master tech, but mine did something similar for about a year and it got worst and worst over the year. I too tried the gas cap trick, but it got to a point were I could leave it alone for an hour, and try a few times and it would start again. I brought it in twice but it would never replicate the issue until I brought it in the third time. It was the fuel pump. When we tested it applying 12v to it...it only work 50% of the time. The mechanic told me that it when the car would start ...the fuel pump didn't have to work as hard or some of the time to keep the car going...but my 95 w124 e320 did have a dual fuel pump too. He replaced just one at first but the issue resurface (he didn't know my model had a dual pump until I told him). So he replaced the 2nd pump and the issue was resolved after a 10months of head scratching. We replaced the fuel filter while we were at it too. Good luck.