2008 Ford Explorer cranks/starts after 90 minutes

Problem is intermittent no crank/no start 2008 Ford Explorer 4.0 v6 103k miles. I’ve tried to have the vehicle towed to mechanic while it will not start, but it starts before the tow truck arrives, within approx 90 minutes. Problem occurs after driving in 70 plus degree weather, but engine is not overheating, Mechanic has replaced starter relay and neutral safety switch, but problem has recurred. Mechanic has suggested replacement of ECM and/or ignition switch. Mechanic diagnosis is that starter is fine but not receiving electricity, however problem has not been replicated while at mechanic in spite of numerous attempts following 20 minutes or so of mechanic driving. Battery is new. Overall, vehicle runs fine, but fails to start without warning.