First, make sure the people you bought this from are fully aware you are still having problems and you are UNHAPPY - don't let your warranties run out. This could still be a bad motherboard, RAM (it does not have to test bad to be incompatible - with each other, and the motherboard), or PSU. I would suspect heat, or a failing power supply - assuming the system is free of malware.
What are your temperatures? The motherboard should have come with an utilities disk that should include a hardware health monitoring program.
If you are overclocking, don't - at least not until this is resolved.
Open the side panel, make sure the interior is clean, and all the fans spin freely. Inspect case interior for additional case fan options. You want good front to back air flow. I prefer one fan in front drawing in cool air, and at least one fan (in addition to any in the PSU) in the rear exhausting hot air out. 120mm fans are much better than the typical 80mm fans as they move more air, but at a lower RPM, so they are quieter. Replace any big flat ribbon cables with round cables, and route all cables to minimize restricting the air flow.
Then blast a desk fan in there and see if crashes.
Make sure your PSU is sized correctly. Use the
eXtreme PSU Calculator Lite to determine your power supply unit (PSU) requirements. Plug in all the hardware you think you might have in 2 or 3 years (extra drives, bigger or 2nd video card, more RAM, etc.). Be sure to read and heed the notes at the bottom of the page. I recommend setting Capacitor Aging to 30%, and if you participate in distributive computing projects (e.g. BOINC or Folding@Home), I recommend setting TDP to 100%. Research your video card and pay particular attention to the power supply requirements for your card listed on your video card maker's website. If not listed, check a comparable card (same graphics engine and RAM) from a different maker. The key specifications, in order of importance are:
- Current (amperage or amps) on the +12V rail,
- Efficiency,
- Total wattage.
You might also run with just one stick of RAM at a time. Ensure you unplug the computer from the wall, and touch bare metal of the case interior to discharge static in your body before coming near the motherboard.
Above all else, do not let your warranty run out. If you have kept your system clean of malware and heat trapping dust, and this computer was sold to you as a gaming machine, it should not be crashing.