Each 5000x5000 pixel image needs ca. 100MB of RAM. Depending on how much RAM your PC has, this might already be the problem.
If you have 1GB of RAM or more, then that's probably not the issue. In this case, you must have a memory leak. Where do you free the allocated bitmap? I see that you unrealize the brush but how about the bitmap?
Note that increasing your swap won't help since that will kill your performance.
Instead of creating a large DC and then blitting a portion of it another, smaller DC, create a DC the same size as the destination DC, or at least the same size as the blit destination. Then, offset all your drawing commands by the (-x,-y) of the sub section you want to copy. If your destination is (100,200)-(400,400) on the source then create a DC (300x200) and offset everything by (-100,-200).
This has two big advantages: firstly, the memory required is much smaller. Secondly, GDI will clip your drawing operations to the size of the DC (it always clips anyway). Although the act of clipping takes CPU time, the time saved by not drawing pixels that aren't seen more than makes up for it.
Now, if this large DC is something like an image (JPEG for example) then you need to look into other methods. One technique used by many image editing programs is to split the image into tiles and page the tiles to/from memory/hard disk. Each tile is its own DC and you only have enough source DCs to fill the target DC. As the view window moves accross the large image, unload tiles that have moved out of the target rectangle and load tiles that have become visible.
Skizz
Make sure you are selecting all original GDI objects to the DCs.
The problem may be that your Bitmap is still selected into the pOutputMemDC when it is being destroyed and one of them or both can't be deleted properly. Thus problems with memory might begin.