I have a problem with my adobe premiere CS4

When it comes to playback, the source monitor provides a perfectly smooth playback of the clip, but as soon as i add it to the timeline and view it through the program monitor, playback becomes jerky, and frames are dropping which makes it far from being really smooth.

I have a Core 2 Duo E4500 2.20 GHz with 3 GB RAM and Nvidia 7100 graphics memory..

I'm quite sure it's not a hardware insufficiency, as it runs smoothly in the source monitor. i googled for solutions but i haven't found anyone with the same problem - most people with similar issues [where playback stutters] can't play in the source window either.

it's quite frustrating, i can't view edited footage before exporting, which means i can't sync background music or anything like that

help would be greatly appreciated!