http://talk.maemo.org/showthread.php?t=50105
http://wiki.maemo.org/How_To_Make_Yo...ry_Last_Longer

uh try these pages and rest there are many to help you .good luck ,dont try reflashing again n again ,it may hurt N900 eyes .
take out the battery and try reboot again ,if you have overlocked the device upto 1 Gh ,then probably thats the reason .
hope it helps