MBWorld.org Forums - SLK/R170 M111 engine cold start problem

I managed to start my Mercedes 1999 E class Estate W210235 (some website call S210) (face lift model 2000, engine M111942) on
1) 3Apr18 started at 757pm (after failed to start for the last few weeks), drove 1 hour 15 minutes stopped at 912pm

Try to start early next day (because ever tried to leave the car for 1 or 2 days and it became very difficult to start)
2) 4Apr18
-started few minutes only at 750am (morning)
-started few minutes only at 1145am (lunch time)
-started for 15mins at 845pm (night)
3) 5Apr18-started for 3 min at 749am (manage to start in one time but a bit reluctant to start)
-started at 720pm, drive 45 minutes stopped at 8:05pm4) 6Apr18-failed to start at 6:20pmI pulled out the hose from the fuel pressure regulator and there is no fuel dipping out. Does it mean the fuel pressure regulator is 100% functioning? What else could be causing this cold start problem?Or, Is it possible that the problem might be due to loss of pressure or air somewhere (loose hose)? I have replaced aircon compressor before. Or sometimes cat or rat get into the engine compartment.
Additional info:
My previous experience was once it started, there will be no problem to start on that day.

I have replaced fuel pump, fuel filter, battery, alternator and spark plugs.

Please help.