Is there any particular develop action that causes this problem? Are you doing a lot of brushing and spot healing? How large are your original images in pixels or megapixels (do not tell me megabytes)? What size is your computer monitor in pixels (width and height), do not tell me inches.
You have a very old (7 years old) and very slow CPU, and most likely it can't keep up.
Also, it's never a good idea to include your serial numbers here in a public forum.