Is one of the cards a "OC" version? When SLI 1st came out if the cards had different speeds they would default to the lowest value. Nowadays whatever card is in the 1st slot is usually designated as the master, so to speak, and both cards will run at whatever speed it's set to.
When you installed the new drivers did you run Driver Sweeper 1st. Sometimes an old file will remain and screw everything up.
Make sure the second card is fully snaped into the slot and nothing is acting as an obstruction.
Your power supply might be getting weak, have you looked at your 12v power output under load? Use Hardware Monitor.