How old is your phone? On your original battery?
I'd say you may need a new one?
There is a good after market one here: http://cgi.ebay.com.au/ws/eBayISAPI....=STRK:MEWAX:IT

Don't go for the cheap ones. They are not what they say.

Edit: I do also get this problem sometimes. My N900 is starting to act up pretty bad. What you can try is to just reboot your phone. (If you have multiboot, do not open your keyboard when you reboot [don't risk flashing a kernel on a flat battery!!!!!!!!!!!!!!!!!!!!!!!]).
At one stage when using the video camera, it went from 54%, to 2% down to 0.1% and then on a reboot back to 20%.

It seems to be having trouble keeping track of the battery power on high usage (CSSU bug?).