I to have had this problem with a couple of PC's. On closer inpection, I did find that the battery clip was loose, but did seem to make contact. I tested the battery voltage and check to see whether it would sustain its voltage, which was all ok.
I did try and contact ECS and ask there technical department about the problem, but an answer was not forthcoming.
I did try to change the battery anyway (to a Duracell one), just in case. The problem cleared for over a month, then came back with avengance.
I personally suspect a fault with either the battery clip or the CMOS NVRAM itself.
However, my supplier didn't seem to have any problems changing the motherboard immediately, although they also had few of these boards with problems themselves.
However, not to be totally negative, I have seen more machines with these motherboards in without any problems at all and very stable.