Windows explorer stops showing PDF’s in the preview pane

Some users in our office use the preview pane in the windows explorer to view PDF's. After a while (~an hour) the preview pane no longer shows the preview.

• On one PC, it says "No preview available" (even though there should be)
• On another PC, the preview pane simply just stays blank.

All PCs have Adobe Reader XI, and one has Adobe Acrobat XI as well.

Does anyone know what might be causing this issue? Currently, the users have to "hot seat" to continue their work.

NOTE – these files all do have previews, as when the users switch PCs, they are able to see the previews that were previously "unavailable" on the first PC.

EDIT: After some investigation, the PC's don't seem to be running hot, the memory isn't running exceptionally high, and the memory seems to be sitting comfortably at ~50%. They are using additional programs at the same time, but that doesn't seem to affect this. (It still runs slowly with out any other programs running)

EDIT 2: File size does not seem to affect the process in any way. A 500 KB file might take 2 minutes to load, whereas a 5 MB file might only take 30 seconds. I also found that during this process, there is 4 instances of Adobe Reader running, anywhere between 4,000 and 300,000 MB of memory. When I stop these processes, if I have a file I am attempting to view, it just automatically restarts the processes, but if I have already loaded a preview, the processes end, but the preview disappears.

UPDATE: After a little digging and research, I have discovered this may be being caused by a buildup in temporary memory, so I have created a dll that (should) empty the unused RAM. — No change.

EDIT 3: Users do multiple things on the PC, like using MicroStation and Photoshop. For most of the process it will sit ~1 GB memory, then unexpectedly spike. Resetting the PC will unload the memory partially, making it crash sooner (~45 mins).

The PC usually sits idle overnight (hibernation) but on the weekend it gets shutdown.