Time to look at temps. (Really wish you had told us your case and the model of the Antec psu - modular?) So I'm reduced to guessing why it works on its side but not upright after 4 months. And I guess dust buildup in a case with a side vent and with marginal airflow for the 295, overheating the cpu when upright.
Anyhow, stand the PC upright, and use CPUID Hardware Monitor to watch core temps. When it shuts down, what were they?
Repeat test with case on its side. Compare temps.
The heat may also be caused by a cpu heat sink that can lean away from the cpu when upright. Doesn't have to be a heavy aftermarket cooler to do that, so check by seeing if you can rock the cooler with your hand.
If its not heat related, we can move on to mechanical issues.