in these cases the first thing to do is always swap the gpu to see if it is the slot the problem or swap the risers to see if it is the risers or the gpu, you can't do much else, also look if in windows it say it working properly or not
Yeah, swap them this doesn't look software related since you mentioned it worked for days and did not said you rebooted the machine in the meantime.The hardware side, power consumption and cooling. If the fanspeed could get raised a little? If not, reduce the memory clock values. Temperature on 3rd card higher than those other twos? Place the coolest running card into the most probelematic slot.
