5D mirror question in AEB mode

I've got a 5D. I bought it used, from a relative. I used it last year, and took nearly 5000 images with it (summer vacation).


Lately, I've been noticing that when I use it in bracketing mode (AEB), Av, +/- 2 eV, ISO 100, one of the shots are completely black, and the shutter speed shows around 1/4000, where the others are what I expected, in terms of shutter speed.


Granted, I was using the camera in about 40F air temp, but I've done this many times before, and never have seen this phenomenon. Plus, I'm starting to see this happen in warmer environments.


The mirror was replaced on this camera, by Canon, and I've checked it, and it is securely attached.


Any clues?