When I open photos from Lightroom, areas of transparency are showing up at random within the photo.
If I convert the Background to a Layer, the problem disappears.
What is causing this, and how can I prevent it?
Win 7 16 gig Ram NVIDIA GT 220
Turn off GPU drawing for now.
Update your video card driver, turn GPU drawing back on, and see if it works -- if it still doesn't work then you need to contact NVidia and help them debug their driver software.
Try changing the GPU setting in Preferences/Performance from Advanced to Normal. You do not need to step down to Basic. That should do it until Nvidia fixes the driver bug. I had a similar problem and fixed it that way. No real drop in performance that I noted so far.
Setting from Advanced to Normal not only got rid of the transparency problem, but my files are loading about 10x as fast.