Any suggestions, other than replacing the camera?
PS How about an external pack?
Older NP-80 batteries have a capacity of 1,100 mAh, with the introduction of the 6900 this capacitiy was increased to 1,300 mAh. So, probably you're using batteries with the old spec, which indeed die very quickly in the 6900. NP-80's nowadays go as far as 1,350 mAh to my knowledge. An external battery pack is no option for the 6900.