After a while I have found the problem... SATA power for the risers... is not a good combo.. So after changing those the rig was stable..
Now with replacing some of the cards for newer ones we have new challenges... ;-) Thanks for the support.. really appreciated.
Sata raisers are better than molex raisers (sata power connectors usually have much less issues with bad connectivity than molex) and are good for pcibus power hungry polarises