BB Curve :: It Keeps Saying Battery Is Drained?

my blackberry 9300 used to last me 2 days with low usage and 1 day with heavy usage. that's the way it has been for about a year and a half and i would always charge it all night anyway regardless of how much i use it. about a month ago, i was watching a video on Internet using the 3g network and suddenly it beeped and said "battery drained, shutting down handheld." there was no battery low warnings or anything, the battery said it was half full at the point. i didn't think much of it, i did a battery pull to get it to turn on again and it was fine for the rest of the day.it kept doing the same thing though, everytime i would watch videos or listen to any type of music. i would literally take it off the charger after a full night of charging and then watch a video on the wifi, 2 minutes into the video or after only one song it would tell my the battery is drained and shuts the phone down even though i apparantly have full battery. the only way to get it to turn on again is if i did a battery pull.well then i thought, ok then, so it only does this with videos and music. it never used, but it does now. but as time progressed, even when i didn't touch vidoes or music and am doing normal non-battery taxing things like texting and phoning people it would do the same. many times i've been in the middle of a call and it would turn off and leave me stranded.sometimes when i reset it, it goes back to normal and it stays on for a while after and sometimes it just turns straight back off again.