Hello,
My wife is having the same problem now with her 1999 ML320. We purchased it new in 99 and have had absolutely no issues until a couple months ago. The car has 98k on it. It started right after the engine belt idler pulley came apart and we had all that replaced . Coincidence maybe but a week later it sputtered and lost power while going to town and she babied the throttle at lights to keep it from dieing to get it back home. Once home she switched it off and it started and ran fine with no check engine lights coming on during the whole thing. A few days later it seemed to turn over a long time before starting but it would eventually start. Now it's hit and miss wether or not it will start at all. After reading the forum I ordered and replaced the CPS with no change. I have since replaced the fuel filter and hoses with the upgraded parts,still with no improvement. I have changed the fuel pump relay with no results but I did notice one thing when testing the fuel pump relay. If I probe the fuel pump relay fuse it shows the pump receives power for a couple seconds when I turn the key on and then it remains on during cranking when things work correctly. The problem seems to be the probe indicates it receives power late in the crank sequence when it takes a long time to start and power "Blinking" on and off during the times the car will not start. Does this tell you anything that can direct me to the most likely cause ?

Thanks so much for any help.