I was out driving with my kids and my husband and the car turned off and will not start

Do you get a "check engine" light? If so, have the code read. Sometimes you won't get the error the first time.
I had a similar problem with a different car, was the cam position sensor. After the 4th or 5th time, I did get a code and that told me exactly what to replace.

check your trans fluid then consider having the trans serviced

You apparently have a leak in your fuel system that allows air in to the fuel system but doesn't leak fuel out enough to be that noticeable. I found this to be true on my 6.2 diesel, so I put light air pressure to the end of a tank vent line and turned on the little compressor. I found the leak spot right away, made the line repair, and have had no more problems with stalling or over-revving.

Well there is no shutoff relay, but there is a fuel pump relay that powers the pump.
I would check it first. The computer grounds the relay for 3 seconds when you crank the engine to boost fuel pressure. Once the engine starts the pump is powered only if you have oil pressure.

check your battery. i had the same problem two months ago. i thought it was the alternator or starter , but the shop said i my battery wouldn't keep a charge.

is there any engine warning lights coming on if not when was the last full tune-up, plugs wires all filter's , if not or don't know ty that.

ok..you must have more than one car???? you don't drive the 300 atall im guessing. #1 start car drive up the road and \back to your house. #2 let it idle till problem starts. #3 once it does start with the problem keep it alive with the gas pedal. #4 once your have it safely from dying in higher rpms "rev it" like a kid would, you know like if you were gonna race some one lol. be careful not to over rev or damage engine. you should do this for five minutes no longer than that. #5 let it idle for another 5 minutes. #6 repeat every day until problem with idling decipates. it took mine 3 300e like 3 days of this. if your not going to drive the car once you get the problem out of the way. turn the car on everyday and let it idle for 5 minutes atleast and take it for a short drive once a week. they get lonelly you know lol. later on you can just turn the car on every 2 -3 days and let it idle but still take it on a short drive atleast once a week

a bad crank sensor can cause this. shuts off while going down road, and will start about 10 min, later with no problems runs fine until it quits again. hope this helps,

sounds like you need a new fuel pump.

It sounds like your Navigator has a vacuum leak after the Mass Air Flow (MAF) sensor. This could be in the intake air tube between the MAF Sensor and the throttle body or it could be a bad vacuum line, bad/stuck EGR valve, bad DPFE sensor, or intake manifold leak. There are several ways to search for a vacuum leak, but I'm not sure this is something you realy are looking to tackle versus having an idea if a shop is being honest with you.

-Rod