The large PSD's could be choking your RAM. Hold down the shift key while opening up the preferences and step through until you see the secret menu. From there you can set AE to purge the RAM after every few frames. It's worth a shot.
I'm not in front of AE right now, I'll do the best I can.
- Hold down the shift key and select preferences
- a sub menu should appear showing the various pref settings (General, grid, etc.)
- select General
- a pop up window will appear, you can 'step through' the all the menus using the prev or next buttons on the right hand side of the window.
- The secret menu is at the end of the menus. You can hit the next button several times until you see the secret menu or hit the prev button once.