My iMac is ALIVE

My iMac is weird. I have recently got an iMac g5 from a friend who didn't use it so he had no problems. I am now having problems now with it. Sometimes it will wake up from sleep mode and will have a black screen, fans will be very loud. and will not respond from anything. When I do a hard shut down (pressing the power button till it shuts off). Once off I boot it back on and everthing works. I recently brought it to apple and they said that my month old hard drive (I replaced it to try and solve this problem) needed to be replaced and they also said to keep a record of when this occurs and this was on the dates that have occurred recently.

3-2-10

3-20-10

4-03-10

4-24-10

4-26-10

5-1-10

5-23-10

5-31-10

5-31-10 (not a typo happended twice)

If you have any ideas on why this happens or how to fix it, I would appreciate an answer.