Intermittents can be difficult. I would get a can of electrical contact cleaner, thoroughly spray both crank sensor connectors, add a dab of dielectric grease to them and reconnect. There is also a connector for the crank sensor in the engine bay near the firewall / #6 injector.

Freshen all electrical grounds for sure. Can't tell by looking at them. Disconnect, clean thoroughly, reconnect. The braided one between the back of the head and the firewall is subject to deterioration on the 4.0

 

Drove my xj back outside left it for 2 hours and no start. Checked the coil for spark and nothing again. I pulled off the computer and brought it inside. Warmed it up over the stove and reinstalled it. Still no start. Checked for codes and found nothing. Checked the coolant temp sensor and the intake air temp sensor on my scan tool and they showed -14 to -17 degrees which is about the same temp outside...brrrr. OK, so now WHAT!

 

Ok, so I was studying the wiring schematics, trying to figure this one out when I found a relay called the automatic shutdown relay in the fuse block under the hood. Well its 10:45pm and I got to go check the connections on it or this is going to bug the s**t out of me all night. Removed the relay and plugged it back in. Hit the key and vrrooom.....HOOORRA AYYY!!!!!

 

glad to hear it

 

More than likely, that relay is starting to "stick". If it happens again, REPLACE that relay. Once they get sticky, they are of no value.