Why does it overheat while it is running programs or is unplugged?

Hi all, i've some problem with an old glorious pb 12" (mac os x 10.4.11 - model a 1010)

while i run programs the cpu, usually works among 50 - 60 ° celsius, get hotter in few seconds, over 75 degrees, then it shuts down.

this annoying problem becomes much frequent while i use my laptop only with its battery: works for some moments, then it shuts down and it restarts as I press any key, and starts again and again until i leave it powered off for an half an hour at least.

by the way if i use it with its power chord it works quite well.

how can i fix it? any ideas? i have to say few days ago, to remove a cd stuck in the drive, this laptop has been opened up: maybe there's a bend on the cpu heatsink copper heat pipe. can this affect the laptop? thank you for your help)