If you are seeing this problem on the camera's LCD monitor, it's not a file transfer issue (such as a poor card reader-to-computer connection, something I've had similar probs with in the past).
In that case, it's either a problem with the camera or the card.
I'd remove the card and closely inspect the pins down in the camera's CF memory slot with a flashlight. Look for any bent pins or signs of an electrical short. Also inspect the problem card, the socket end of it, to see if there is any damage such as a pin broken off in one of the sockets or damage around any of the sockets. It is possible for memory cards to work partially, even with a damaged slot in the camera or sockets on the card. Not all those 40 or so pins are used and there are multiple paths for data to record to the card. But under certain conditions, a damaged slot or socket causes image issues. Particularly during heavier usage. Unfortunately, these also can be symptoms of other components in the camera starting to fail.
It also might be a "mystery fault" with the card that only shows up on this particular card. Try formatting the card in your computer, then again in the camera before using it. I've had "issues" with a card that a friend borrowed and used in her Nikon, after she returned it. Formatting in computer and again in camera solved the problem.
There are some memory card test apps available. One I have is part of a Lexar image recovery software. It's an applet within that software that tests and repairs memory cards. It's not uncommon for cards to have bad sectors... occasionally right from new, sometimes developed over time. Nearly all manufacturers test cards when they come off the assembly line and "map out" any bad sectors, flagging them so the camera won't use them. All cards have enough extra "head room" in them that a few bad sectors mapped off in this manner don't reduce the size at all. So you never know they are there.
If one of those sector flags fails or a camera doesn't recognize it for some reason, it's possible the bad sector might be used, giving problems. That's where the testing and repair app comes in handy.
You mentioned it was a Sandisk memory card. Those are usually quite reliable.
I'd be trying a different memory card extensively to see if it gives similar problems. If it does, there's a fault in the camera.
You might try rebooting the camera.... detailed instructions about doing that here in this other post.
You also might try a firmware update. Neither the reboot or the update are likely to solve the problem, but they are free and easy to try yourself.
If those don't help, then it's most likely a component in the camera failed.
Was the camera bought new or used? If new, I'd be sending it to Canon to have it checked under warranty. If used, well it depends.... if bought cheap it might be worth paying for a repair. If not, I might be talking with the seller about returning it for a refund, if that's an option.
. No answers though O_o[/edit]
Remember, this is from info that I've read in my travels surfing and by no means scientific fact (although there was one or two official shootouts).
