1D Mark IV Exposure Oddity

I have a relatively new 1D Mark IV (still under warranty with about 9500 shutter count). Today when shooting sharp tailed grouse in the high speed drive mode, if I shot a burst of two images in a row, the second image would have a shutter speed 1-2 stops slower/ overexposed. This only happened when the camera was idle for about 5 minutes. If I was using the camera continuously, the exposures were fine; in a burst of 2-10 images all would be exactly the same exposure. If I shot bursts then waited for a couple of minutes between the bursts, again the exposures would be identical. But if I waited for 5 or longer minutes between the bursts, the first image would be fine, the second image would be overexposed and remaining images in the burst would be correctly exposed. I wasn't using exposure bracketing. Camera was in Av. I tried two different lenses and got the same results. I reset the camera to its as-shipped configuration and still had the problem. I took out the battery and reinserted it and still had the problem. I've got another couple days of shooting before I can call Canon. Any suggestions what I can do in the meantime?

Thanks.