Just done a 60 mile round trip & so far so good! no reboots Will try again tomorrow with the addition of the SDHC & extra maps.

I personally think the rebooting issue may of occured because:

1. Windows prompt wanted to scan and fix errors when plugged into the USB port & I selected 'YES' to fix errors.

I discovered this happens when pulling the USB cable out of the Nuvi or PC, instead of using the recommended method of ejecting the unit which is to use the "safely remove hardware" feature in Windows (maybe able to dissable feature)

2. Nuvi was formatted by Windows right click format/FAT32 to stop annoying prompt's which worked, but at the cost of a working Nuvi.
It remained a mystery to me before posting on noeman mainly because the reboots were totally random & had no pattern hence thought it may of been a hardware fault eg, dry joint on the PCB.
So a big thank you to everyone who helped me! much appriciated.. Feel's great to have a working Nuvi again