Shuts off when battery drains to below 50%

MacBook Pro
Intel Core 2 Duo @ 2.33 GHz
2 GB RAM
120 GB HDD

Anyway, I've had my MBP since January 2007, and the battery already failed once (flashing "x" instead of the battery icon problem), so it was replaced under warranty.

But now that it's out of warranty, as of maybe 1 month ago or less, whenever I'm using it on battery charge, it will suddenly shut off (doesn't go to sleep like it used to, shuts off just like that).

According to coconutBattery, the original battery capacity was 5500 mAh, and it's now down to 2901 mAh (52%).

What's going on? Is it time to replace the battery (AGAIN) ?? Any ideas would be great, thanks!

im having the same sort of problem with my macbook, i have 3948 mAh left in it but as soon as it his around 25% it just shuts off i tried to calibrate it but with no success because its not completely drained or something like that so just letting you know you're not the only one but have you tried the battery calibration? http://support.apple.com/kb/HT1490 will be interesting to see if this works for you, but i still have to find the time to try it again, when it goes dead, plug the charger in and start it up and see if it shows any percentage of charge left, with me it did which was frustrating but i normally leave mine plugged in now.