Please help r9 280x by mining zec, lower your H/S in half

Hi guys, I have a rig, with 6 r9 280x, but since a few days, a card, lowers its hashratre to 150 H / S, when the normal of these cards is above 280 H / S.

All cards have dummy, to maintain H / S,

For mining use:

simplemining & claymore

Thanks for your help.