Exclamation [Q] Severe Battery issue with galaxy SIII.

I am facing a severe issue with the battery of my samsung galaxy SIII which i have purchased in june-12.
Sometime battery of my phone is charged very quickly( max 1 hour) & then it will discharge very quickly(max 2 hours) too. Battery will completely discharge from 100% to 0%(in 2-3 hours).
This problem occurs once in 10 time.
I have observed, When my phone battery start charging very fast, from 10% to 85% in just 20-30 minutes.
i disconnect the charger, Restart the phone & it is showing only 12% battery remaining.
It means phone was not charging just the battery percentage is going up.
this problem comes to me at least once in week. and i have to charge my phone 4 times a day.
Normally, I can use my phone battery for full day once fully charged.
So, it doesn't seems to be a battery issue.

Please have a look in the issue & guide me how to proceed.