replace all of your risers
do you have any missing device drivers in device manager?
ya man, test each card till error appears, replace riser on the faulty one may fix it (hopefully) most risers have a terrible fail rate, sometime right OOB, sometimes after a few weeks.months mining

i run also a free iobit softwear called "driver booster". free and works. it usually finds drivers for "PCI-E" stuff, and have had it help having all those updated
keep in mind you DONT need all the audio drivers for each card installed, as they will just eat up resources
Here's the thing. I plugged GPU 1 and 2 in and it crashes. Then I try GPU 1 by itself and it works. GPU 2 by itself and it works. So it cant be the risers can it?
haha damn thing, rigs are so finicky sometimes :/ maybe theres a PCI-E port acting up on the MB ? switch around the slots you plug the cards risers into, see what happens

make sure no dust in the slots also
just keep trying to isolate it,, maybe you can get it to 5 cards, and go from there
and update your system drivers! it could be windows just confused by all the cards
also: if you get the time, you can try running a linux OS, then you will know if it works fine on there its just windows being a bitch

SMoS, or Hive, NvOS even