Macbook slow and shuts down when battery indicator is not empty

Hi everyone,

I have a 17'' macbook pro bought mid June 2007.

Three weeks ago I noticed that my macbook pro would shut down at 20% battery (if I was not plugged) and would not start back until I plugged it in. I figured it was a battery problem. However, my macbook started getting really slow with the little 'volleyball' icon appearing more and more often without having to use lots of different of programs in parallel.

Also last week I realized that now my macbook pro shuts down automatically at 67% battery! I checked the little button behind the battery and when this happens it shows to me 3 out 5 light indicator. So I'm guessing that the battery is not dead and yet my computer shuts down.

I resetted my SMC (I think that's what it's called) and repaired my disk... I have not reformatted it though.

Can someone help me out? Is the fact that my mac is slow related to its shutting down prematurely?

For info: my battery is 2y old and has 360 cycles.

Thanks a lot in advance for your help!

Cheers

aqualover