PSU is from Thermaltake. I have one mobo that can run up to 5 cards, I guess I will try it, but I've changed the risers connection on the mobo, as it can have up to 12 gpus I was able to change them all without success.

One weird thing is that I wasn't able to make it work properly using the jumpers (even if it powered all cards), but with the adapter it gets stable sometimes at first try and sometimes after reseting a few times.

Since 2x gpus and its risers, mobo, 2x mobo molex are connected could it be that a failure while initializing other psus could overcharge the main psu, as it doesnt have too much spare power? Note that the rig resets as soon as the mining is about to start (miner software is on), so it's as soon as all cards are really powered up ready to mine (with all cards detected), could it be that a late power up causing this overcharge? - As I said, if I set a dummy miner to start, the rig 100% of times gets stable at first try, because the cards aren't really powered up. If it was the mobo, wouldn't it not occur like this?