Does this occur without regard to where you view the pictures? Make sure it's not in the device you're playing them back on.
I would look at two things. First of all, either enable the "overview" function in your camera (that's what it is in a Nikon, not sure what it would be called in a Canon, but it will be the one that gives you information and histogram), or use a computer program that provides the EXIF information on the images, and see if there is some common factor to the ones that are bad. Although it's unusual, it's possible that a shutter problem could be at issue, and if so (wild guess here), I would expect the problem to occur either always above, or always below, the camera's sync speed, which is the speed at which the shutter's mechanical behavior changes.
The second thing I'd look at is the memory card. Try a different memory card, and see if the problem occurs. Corrupted memory cards can cause all sorts of odd problems.
Comments
Does this occur without regard to where you view the pictures? Make sure it's not in the device you're playing them back on.
I would look at two things. First of all, either enable the "overview" function in your camera (that's what it is in a Nikon, not sure what it would be called in a Canon, but it will be the one that gives you information and histogram), or use a computer program that provides the EXIF information on the images, and see if there is some common factor to the ones that are bad. Although it's unusual, it's possible that a shutter problem could be at issue, and if so (wild guess here), I would expect the problem to occur either always above, or always below, the camera's sync speed, which is the speed at which the shutter's mechanical behavior changes.
The second thing I'd look at is the memory card. Try a different memory card, and see if the problem occurs. Corrupted memory cards can cause all sorts of odd problems.