Well 3d Mark is good benchmark but the thing is that probably not making the most pressure on your GPU especially if you did the test in just 1080p.....Even if is it old I will still suggest
Heaven Benchmark just choose higher resolution and details and see how that goes....Also
Furmark will make your GPU to work on constant 100% load but do not use it for long time 5-10 min at most.....
P.S.One more thing maybe you actually have the problem when CPU&GPU is under the heavy load because then your PC actually using the Maximum Wattage and making the most pressure on your PSU and can not provide that amount of power anymore....So to be absolutely sure if that is your problem you can start Furmark in combo (at the same time) with the Cinebench or some other Cpu benchmark and then your PC will use Maximum power....If it restart you will know that your PSU simply can not handle that....GL