GPU is crashing and becomes unresponsive. No thermal control, which is why the fans run 100%. First thing I would look at is power. It looks like your power supply doesn't have the 12VHPWR connector, so I assume you're using the adapter? I would reseat it, make sure it's fully seated.
Depends on the temps. A lot of these new cards aren't turning the fans on until it hits 60C, so if you're hovering around there it could turn off intermittently.
Given that it seems to work fine in the store, I would stick to troubleshooting power. But don't stop at the PC. Consider the power cable, make sure it's the one that came with your PSU. Replace it if possible. Check the outlet as well. If it happens at home and doesn't happen at the store, an environmental is the most likely culprit.
Submit photos and a description of your PC to our build showcase
See other custom PC builds and get some ideas for what can be done
Services starting at $149.99