fuel pump;filter mod;CPS?

[nisan17]

The scenerio goes like this for my 98 ML320, 168k:

Beginning of August, no power during passing, then no starting. Indie says low fuel pressure, (1)changed out the fuel pump. Ran good for a few miles and starts and the same thing, Hard Start, no start. (2)Did the fuel filter mod, wasn't done at all previously. Ran good for a few miles and starts, then again hard start no start. (3) Changed the CPS, then a few good miles and starts, then again, hard start, no start.

What could possibly be going on? It appears to be repaired then the same symptoms!!?? It's getting very disheartening.

Thanks in advance