Some of my video thumbnails don't show

Question

I triple back up my photos and videos on external hard drives.
Recently, I went back to look at some of my May 2012 home videos on my back ups. Three of the videos didn't seem to work. The thumbnails didn't show an image from the video. Went I went to play them, all that happened was a black screen with the timer at the bottom of the Quicktime player showing that the video was playing, but all that I could see of the video was a black screen and no audio. I went through searching through previous iPhoto libraries in time machine and eventually found the video footage in a 'masters' folder (they still didn't show up in the iPhoto library - I had to do a search on my Mac).
Having rescued all the missing videos, I thought the problem was over. Sadly, no. Another time I went back to look at the videos on my back ups, suddenly a whole load more videos were behaving similarly: no image from the video for a thumbnail like the working videos have; pressing space bar to play them in Finder led to the black screen I'd seen before. This time they played in Quicktime, though after taking ages to load up rather than the split second it normally takes. But still the missing thumbnail and the mysterious black screen when I try it in quick view in Finder.
The same thing happens on all 3 of my back up external drives, so it can't be a problem with them.
The same thing happens on my white, Snow Leopard Macbook AND my silver Lion Macbook Pro.
So I don't know what the problem is!
Another thing, (which may be related), recently when opening iPhoto it says "There is a problem accessing one or more files in your iPhoto library. Do you want iPhoto to repair permissions for this library for you?" to which I reply 'Repair'.
These videos are of great sentimental value, as you can imagine. I don't want this to keep happening. Any idea why this happens and how to stop it?
yours.

Answer 1

"Some of my keyboard keys don't work"
So exactly which keys don't work?
So like Grant says, "external USB keyboard".
I have the identical machine and the "qwertyuiop[]" keys stopped working. I thought it was a problem with the connector. After using it with an BlueTooth keyboard for a year I bit the bullet and bought an entire upper case on eBay for $69 and installed it myself. I actually just bought the keyboard itself but there are ~70 tiny screws that hold it in and I didn't want to go through all that to have to put it in. Those upper cases are very cheap now so that was the way to go.