Video editing is resource intensive; a gaming-capable PC produces best results. That said, try these measures...
Reduce VP preview resolution. (Right-click in the preview window)
Allow the green line above the timeline to complete before previewing.
Toggle the Hardware Accelerated Video item under OPTIONS|EDITING, to see if that makes a difference.
If file sizes are large, employ proxy editing. For details see the 'sticky' post at the top of this forum.
Close all other apps/programs.
Use the latest VP version. (If your current version is registered, retain its installer.)
Upload your project and link it here if you'd like someone to check it out. Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/