This is a hardware malfunction causing file corruption.
It could be any piece of hardware used in transferring the photo from camera to Lightroom, which begins with the camera card and end with computer memory.
You need to repeat the import with different hardware, and if possible on a different computer.
I agree 99% with your assessment. Basically your assessment is that it isn't Adobe Lightroom, since the only other possibility is pre-import file corruption due the factors you mentioned (plus, perhaps hard disk problems which I have not noticed). The only reason I don't agree 99.9% with your assessment is that the only corrupt files turn out to be DNG files. Perhaps a few corrupted bits do not matter to the JPG format as much as the DNG format. I also believe it was just coincidence that I first found corrupted DNG's immediately after upgrading from Lightroom 4 to Lightroom 5 and importing some new photos. In fact I discovered lots of corrupted files over time and since I did not discover them all at once, it seemed like the corruption was ongoing. Thank G-d I shot with DNG+JPG since even if some of the DNG's are corrupted, I still have the matching JPG.
I went back and played with the DNG's that Adobe Lightroom did not like with my Ubuntu software mentioned above. I discovered that while they show the thumbnails of corrupted DNG's just fine (unlike Lightroom), they can't open corrupted DNG files. In other words, Lightroom Adobe was correct in saying the now known corrupt DNG's were corrupt. One question I have is why Lightroom could not discover the corruption when doing a "DNG Validate" under the Library menu. From what I know, the checksum for the DNG is stored in the DNG and the stored checksum would most likely be not equal a corrupted DNG's checksum.
I am going to operate on the assumption that your assessment is correct. However, this still does not solve my problem. I realize you can't help me much without actually having access to my physical equipment. I have decided to buy another 64GB SD flim card so I can use them for backup for a longer period of time before I am forced reformat them to make room for new photos. I also plan to maintain detailed written records of the when's, the how's and the what's of my data transfers from my camera to my hard disk.
Thank you for the clarity you have given me. I've used LIghtroom for a long time and until this Fall, I always found it rock solid. It most likely was rock solid this Fall too.