Second that, sure it is the battery, i had the same problem with my old Nokia, the batterys are great, i recon i charged mine around 600 or so times as i killed it each day as i use it alot, so i think thats good, Nokia say the like is variable but expect around 360 charges, so one a day for a year.
One trick i found towards the end was when it says fully charged, take the charger out while the phone is off (charge the phone off) and then plug the charged back in. I found that i would get the full message, then if i took the charger out, put it back in a few minutes later it would charge again for 20 mins and then say full, and i just repeated this, and it seemed to work ok, the like was never the same, but i got a fair few hours out of it.