I'm trying to combine three pictures.  There is some overlap among them, but I need to shrink and rotate them to make the common elements match up.  I took the middle picture, expanded the canvas size, and then tried to fit the next picture to it using a new layer.  The problem I'm having is that PP X3 allows me to move the 2nd picture only once, but then when I go to move it again, it freezes.
I've provided information that might be relevant, and if more is needed, I'll do so.  I apologize if some of it is irrelevant.
The pictures are large—the first one with the expanded canvas is 130MB, and the 2nd one is 30MB.  But I have a quadcore  CPU, with 8GB Ram, and am using Win 7 Home Premium, which is supposed to allow me to access 8 GB.  
In the interest of saving memory, I turned off History.  When I load the first largest image, Resource Monitor initially shows 15% Physical Memory usage, which then settles down to 12%.  When I open the 2nd image, copy it, close it, and load it as another layer on the first image, Memory usage goes to 24%.  When I move this layer once, CPU usage spikes at 31%, and then drops down to 2%, and physical memory use is at 30%.  When I try to move the layer again, CPU spikes to 25%, and then, after about 30 seconds, drops to 2%.  Memory jumps to 30%.  PSP appears to freeze—the layer does not move, and first there is the cross arrows of the Pick tool, but then it turns to a red circle with a diagonal line.  Eventually, I have to close down PSP with Task Manager.
Why won't PSP allow me to move the 2nd layer more than once?  What can I do to overcome this, short of reducing the picture size?
TIA.