1. Overheat due to faulty / blocked fans
2. PSU voltages
3. Household power surges fridge etc switching on/off
4. faulty memory stick try memtest click here
5. virus or spyware scan for both
6. driver conflicts especially grahics cards get the latest drivers for your equipment
1. Works for 6 to 8 hours before hanging may mean over heating.
2. Psu could be struggling any idea what size PSU (wattage)
The second shop said they found a faulty BIOS reading and it may be the motherboard.
Any idea what this reading is?
set the BIOS to default settings.
You could always flash the BIOS as a last resort.