97 SOHC 4.0 kills or won't start

97 Explorer with 75,000. Always been reliable in the past. Last 6 months,
mostly a hot spare. Full tank of gas, Stabil per directions. Used once
every other month. Battery charged first of every month.

This week drove fine on Tuesday. Loaned to Dad on Thursday. Drove home
fine. Made some stops, and after one stop, it coughed and hesitated after
leaving a stop sign, making a left hand turn. Next stop, would not start.
Turned over. Seemed like no spark, but heard several muffled explosions, so
at least a few cylinders seemed to be sparking.

Called for a ride, and heard from my father that after he ran his errand
with it and backed it into his driveway, it wouldn't start after unloading.
He waited an hour, and then it started.

I waited a few hours, got a ride to the parking lot, but it still wouldn't
start. Tried again 3 hours later. Still wouldn't start. Went back Friday
morning before having it towed. It started like normal.

I drove it to the nearest auto shop and explained the problem. They called
back last night and said they can find nothing wrong. No charge, come and
get it. I left the lot and half a block away is the same corner it
hesitated at on Thursday. Took the same left turn from the stop sign, and
this time it died in the intersection. After coasting to the side of the
road, it started back up and I returned to the auto shop.

They still can't find anything wrong. Anyone have any ideas?