On what computer operating system is your Premiere Elements 12 running on? Have you updated 12 to 12.1 yet? If not, please do so by using an opened project's Help Menu/Update.
Not enough information in order to help you.
1. Properties of source media (including file extension and brand/model/settings of camera that recorded the video).
2. Is problem restricted to one particular file in one particular project? What happens if you start a new project? What is the
duration and size of the project when performance is slow and freezing sets in? What are you doing in the program when this happens?
3. Using a pen and tablet device instead of a mouse?
4. Does problem exist if antivirus and firewall(s) are disabled?
5. Please describe computer environment, including dating on video card/graphics card driver version. Did you install the program to the
default Local Disk C? Where are the Scratch Disks directed and how much free space there?
Lots more questions, but let us start here and then decide what next based on your reply.