Current verizon mifi cuts off when in hot car. Panasonic toughbook is still working, mifi off and won't turn on until cool. Unable to find a ruggedized version?
There are noted heat limitations for these devices. I have had a similar experience with this situation and a few MiFi models.
The only solution available is to try and store your MiFi in a cooler location instead of out in the open where the sun will bake it. You might want to try hiding it in a glove box, center console or perhaps even in a small cooler with an ice pack while the device is not in use. That way when you do need it, the device will be at the proper opperating temperature and fire right up.