Sometimes when you are working on a project in Photoshop or Elements, the software slows down to a frustrating crawl. To clear up this RAM problem, select Edit > Purge from the Menu bar. In Elements, choose Edit > Clear.
This opens the Purge Menu where you can choose Undo, Histories, Clipboard or All. (In Elements, you can choose Clipboard, Histories or All.)
If you have used a copy/paste function, try clearing the Clipboard. If an unusually large image is there, that may be the source of your problem.
You can clear out the Undo and Histories, but be sure that you are at a point in your project where you will not need to use the Undo feature or the History states to undo an edit. Clearing these eliminates the ability to undo what you have already done, but can free up more RAM and speed up the program.
Once you have purged, the Histories and Undo will begin building from that point forward.
With any luck, that will solve the issue so that you can continue to work on you project without shutting down and restarting.