Please help! My iMac cannot play videos without crashing!

Question

Hi there, I've had my iMac for about two years, it was a gift given to us. In the last four months it's begun to have issues playing videos. After 30 minutes the videos start to lag and chop and the sound will not play either; the sound becomes very grainy and then it just starts to lag a lot.
Everything else works fine though, surfing the web, using photo-shop, etc. I should also mention that this dose not occur when I stream or listen to music, although, at times it does chop, but after 2 minutes it stops and plays the music fine.
The Mac was a gift as I mentioned before, it dose have a lot of programs and files on it from the previous owner; I'm not sure if perhaps it's just a matter of the hard-drive being over loaded or not
If anyone out there has the tools or knowledge to fix this, that's be greatly appreciated.
Thank you very much,
Have a wonderful day,
Mitch
Also, if any of this model information helps please take a look;
  Model Name:          iMac
  Model Identifier:          iMac7,1
  Processor Name:          Intel Core 2 Duo
  Processor Speed:          2.4 GHz
  Number Of Processors:          1
  Total Number Of Cores:          2
  L2 Cache:          4 MB
  Memory:          3 GB
  Bus Speed:          800 MHz

Answer 1

Hi Mitch ..
What kind of video?  Are you viewing YouTube video?