@zlatko, You have either broken SW or broken HW on your device, as your stability experience is so different from mine or @late88's.

Have you tried to restore the device to factory settings and then re-upgrading it to 1.0.3.8 ?
Unfortunately AFAIK you have to go from one upgrade to next in steps, there is currently no way of directly upgrading your device from the fatory-clean state to the latest SW level.