hey.
i got the same problem after more than a year of using my w810i (hadn't taken so much photos, just ocassionally) ... I suppose that the problem is caused by some electronical issues.. i mean the button is having problems with contacting the electronic paths or somewhat like that :P i think that replaceing it with a brand-new one should help.. (if it's possible to get just the button ofc.. :)
Nevertheless i can't be sure about it cuz i have already given the phone to my bro and bought a new one so i don't care about it now :D