Q: Overheating, harddrive failing

I have a a G4 powerbook that overheats and locks up. The lower bank of RAM has failed. When it it cool, I am able to boot it up with internal hard drive. After about 15 to 20 minutes it will stop functioning. I am trying to decide what I should do -as in- try to replace the fan (after I learn how to check its functioning) and or replace the hard drive, or try to run it off external hard drive. This I am attempting to do, but I am trying to get the contents loaded and it has heated up before all the info has tranferred via superduper. It only started with this problem in the last month or two. I have had it since 2004, and it has had a hard drive replaced once already. Hardware profile has error on the ram as noted above, but no other errors show up when it was run. What am I missing