Do you run with background rendering enabled? If so, have you waited for background rendering to complete (all of the "green" above the timeline to go away)?
If you do a "File/Delete Auxiliary Files" for the current project, and wait for background rendering to finish, do things settle down?
What some folks tend to do on machines that aren't "fast enough" to keep up with high-def...they edit fairly quickly with background rendering OFF, then when they're ready to review their work smoothly and in real time, they turn background rendering on (*maybe* doing a File/Delete Auxiliary Files), and wait for everything to finish background rendering. Then, they can review what they've done without hitches or dropouts.
BittMann