2002 5.7 Intremittent Start Problem

[explorer2]

Newbie here, so apologies if this topic has been covered. My 2002 Savana has on three occasions not started without a lot of cranking. Twice had been shut down maybe an hour, once after sitting for several days. After the second time, I had a technician check for fault codes, change the fuel filter, check fuel pressure, and flush the injection system. All was well, then after not being started for 4 or 5 days, would not start without a lot of cranking. Any suggestions as to what to look for as a possible cause? Runs excellent otherwise with no issues,the three incidents have occured over a period of two months. I just want to get to the cause of the non start issue and quit worrying about any surprises. Mileage is approx 29000. Any input is appreciated..thanks,