Since your rig has been working for 2 years, try to unplug all cards but 1, does it mine now? if it does, plug in another one and keep on going until it crashes.
when the rig crashes at X cards take the newly added Card, put it away and remove all the cards in the rig, now take the card you just put away and put it in your rig, does it mine ? if not then it's the card, if the rig is mining with that card it might be that the PSU has lived it's life