Macbook Shutting down randomly on battery

Hi Guys.

I'm running a macbook, 1.83Ghz Intel Core 2 Duo (HD 320GB, 2GB Ram) running Mac OSX 10.5.5.

This is a well loved, but well used machine. It is in near constant use and often burning and ripping DVD's. I have over 10k Photos and 160gb music which is constantly in use.

The problem is that when I disconnect from the mains the computer switches off. It's not a battery issue (I assume) as I can get a couple of hours from it when it is off mains but it still turns itself off in random intervals. It is not related to (as far as I'm aware) any specific program(s) and does this when iphoto / itunes / safari (main 3 uses of my puter) is running. The intervals that it switches itself off are random. Tonight It turned off after playing 1 album in itunes. I rebooted and carried on playing itunes for a while then reconneceted the mains, then it didn't shut down, I disconnected the mains and it instantly shut down?!?

Any suggestions?

Warmly

Jeshua