These type of intermittent problems can drive you nuts. It's far easier whenever something just queefs out. Anyway, in my experience the problem could be a solenoid which has become sluggish from the crud which accumulates in them over time. The former tube in which the solenoid slug operates can also become scarred from wear and cause the slug to bind. Heat soak seems to cause a reluctant solenoid to stall while it will work normally once it cools a bit. Whenever the starter balks, try tapping on the solenoid with some type of small battering ram (such as a 3 foot long 2X4, a 2X2 piece of rock maple or is just about ideal) while someone turns the key. If it starts, you'll know what to repair.

Another possibility is that the contacts of the ignition switch have become less than predictable. Unfortunately, the best time to try to isolate the cause is the next time the car does not crank but before it is jumped and that might not be very convenient.

Also, check all the conductors and terminations to make certain that they are sound and secure; really, that's the thing to do first.

Rich