Macbook Pro Randomly Shuts Down When Unplugged

Hi everyone, I used to just lurk here, but I've run into a problem I can't solve.

My 15" Macbook Pro has very recently decided to just shut down at random when it is running on battery power. I bought it in June 2007, and its battery was not part of the recall program. It does not even shut down at consistent battery percentages; it has not shut down at 90%, 47%, 72%, etc. etc. There is no error, no X on the battery icon, just a hard shutdown. I'm only at my 120th battery cycle, so I know it's not a matter of it being old, but here's my information:

System Power Settings:

AC Power:
System Sleep Timer (Minutes): 60
Disk Sleep Timer (Minutes): 10
Display Sleep Timer (Minutes): 5
Automatic Restart On Power Loss: No
Wake On AC Change: No
Wake On Clamshell Open: Yes
Wake On LAN: Yes
Display Sleep Uses Dim: Yes
Battery Power:
System Sleep Timer (Minutes): 5
Disk Sleep Timer (Minutes): 10
Display Sleep Timer (Minutes): 1
Wake On AC Change: No
Wake On Clamshell Open: Yes
Display Sleep Uses Dim: Yes
Reduce Brightness: Yes

Battery Information:

Battery Installed: Yes
First low level warning: No
Full Charge Capacity (mAh): 1688
Remaining Capacity (mAh): 963
Amperage (mA): 880
Voltage (mV): 12533
Cycle Count: 120

I have tried everything I know to do here - I've zapped the PRAM, Disk Utility-ed my heart out, made sure that the energy saver options weren't tricking me, and installed SMC fan control to make sure I wasn't overheating (I'm not). Any help on this would be greatly appreciated, since I'm a graphic design student who just got back to school and needs full computer use ASAP!!!!