2000 Chev express van intermittent no start

Need some advice:
2000 Chev express van 5.7L gasoline vortec engine. had no start problem, toewd into shop, in tank fuel pump replaced (fuel pump relay too) all Ok for 4 days. no start, just turn over. tied 4 or 5 times cranking engine, finally started. went back to shop, checked fuel pump Ok, prssure Ok 65PSI. scanned for codes. no codes. mechanic started & shut off van 20 times, no problem. took van home. next morning tried to start, engine cranked, no start. shut off key. tried to start. no cranking, no engine turn over (like battery disconneted). turned off key tried again 3 times finally cranked & started. took back to shop. van has been at shop 3 days. no codes and starts every time. any suggestions. van has new plugs, cap, rotor, wires, fuel filter. battery terminals cleaned, battery load tested fine. Mechanic say he has no idea and cannot fix until it fails. can't afford many more tow calls
thanks in advance for any ideas.

Just to clarify, is the problem that it won't crank over or that it will crank over but not fire up? In the first instance, assuming the battery and cables are good, My first guess would be a bad starter solenoid. In the second instance, assuming all the ignition electricals are good, the fuel pump relay or fuel pump it self may have an intermittent fault.

you need to catch it in the act.
When no start check for fuel pump running and proper fuel pressure.
Check for spark to all plugs.
Read post below about the fuel pump wiring problem on cross member.