I added knew maps to the sdhc last night & the nuvi could not see them..
I checked & the images are complete byte for byte

Today I used the 3790, I thought It was fixed, & then the sudden crash!
But this time it did not reboot, I had to manually power up!
It happened when it was re-calculating the route as I purposly ignored a turn...
It never seems to crash when a destination has not been given, which kinda indicates it is a memory issue.

I dont know if this could cause issues but In tools/settings/navigation/car/map detail.. i set to more &
map data layout i set to more data
could the nuvi be struggling with rendering at that level to cause a crash?

view map/speed/SW0mh/shutdown causes/data abort.."log", then scrolled down to general registers
the CPSR is in red font 0x6000003F
At the bottom,
Symbols status: stack trace incomplete.. anyone know what that means?

Just backed up everything & want to format the Nuvi & SDHC using the RMPrepUSB tool Lollypop000 suggested!
It looks very complexed, is there a tutorial on how to use this tool? can it do an unconditional format?