|The most common cause of shutdown after a system has been runnig a few minutes is overheating. |
Your motherboard BIOS has some settings that force shutdown at a set temperature.
I suggest you download, install and run CPUID. That free program can be accessed in real time to see if overheating is the issue.
Have either of the repair places opened your case and looked inside?
Get CPUID at the link below. Get the hardware monitor.