Premiere Elements 7: Preview playback problems?

[evil-authoress.livejournal.com]

Hello, everybuggy! First time poster, first time video maker, the whole thing. I recently bought Premiere Elements 7, and I'm having trouble with the preview playback as I'm working. The audio is fine, but the video goes in fits and starts, sometimes playing at half speed or not at all, so obviously it's hard to tell how the clips are fitting together. I've tested the source .avi on its own, and it seems fine. Any thoughts or advice would be greatly appreciated!