choppy video

okay here is a question...I have a powerbook, g4, 1.5 GHz, 1.25 GB of RAM. I am running Leopard. I use a Seagate FreeAgent Pro 750 GB for all my video editing. Some imports that I have been doing lately are all choppy. I took an old file that I knew worked...it's playback was choppy. I saved the file on a different external HD...same thing. I saved it on my local HD (80GB with 25 GB of free space), same thing. What do you think is up?