I have an IMAC 2.1 GHZ Power PC G5 with 2.5 gb ram. Safari 4.0.3. Recently I have been unable to run movie trailers and some video clips, even from YouTube, smoothly. The audio is OK but the video runs in a jerky manner suggesting it is skipping video frames, even the scroll bar moves in a jerky manner. My small clips in IPHOTO run fine and most YouTube is OK. Any ideas? All appreciated.