Anyone know the problem?

[PMillz]

Hi, i have a iphone 3GS on o2 (UK) it would turn itself off after about 5 mins of doing anything, i was told it could be the battery so i changed it, i turned the phone back on after reassembling it, and it worked fine for a hour or so until i went to put it on charge, when i put it on charge i went to bed and left it over night when i left it it was at 24% and when i woke up it was at 18% despite being on charge all night, i changed the charger and noticed it was turning itself off every few mins and was showing as charging but the % was not going up, however when it reboots itself the battery % would fluctuate first time it was 18% then 52% then 20% then 59% anyone know what the problem could be?