From what you've told me, there's 2 possibilties.
1) If you're clicking the "Preview" button, or "File->Preview Project...", and it opens up a separate window.
In this case, it's most likely due to the complexity of the sequence. That window doesn't buffer at all, and a high resolution / framerate (down the bottom of the window) or complicated effects, or many tracks, all can cause what you're describing. In this case, unfortunately the only options are to either lower the resolution / framerate, or (if that doesn't help) just assume that the sequence preview is good enough. Normally I would suggest you export at a lower resolution, but the free version tracks your export count.
2) If you're just hitting play in the main window
In this case, there's a few options.
You could try clearing the cache (Open a blank project, then go to "Tools->Options->Disk (Tab)" and click "Clear Unused Cache Files")
You could also try lowering the preview resolution (Right-click the preview, select "Change Preview Resolution...").