Troubleshooting a Canon S5 IS

[Soonershooter]

I recently purchased a factory-refurbished Canon S5 IS camera and I am having a strange problem. The camera may be defective, but since I am not very familiar with this particular camera, I'm concerned that I might have inadvertently caused this problem.

Here's the problem. When I shoot a picture at ISO 80, 100, 200, 800, or 1600, I have no problem. However, when I shoot the very same subject at ISO 400 the photo comes out quite dark. This camera displays a histogram just before the photo is taken. When I am using ISO 400, the histogram of the actual photo is shifted to the left by about 2 stops from the histogram that was displayed just before the photo was taken. This seems quite strange. When using any other ISO setting, the before and after histograms look identical.

This problem with ISO 400 occurs regardless of whether I am using Av, Tv, or Manual Mode. I am not using flash, and the Exposure Compensation is set to zero for all photos. I have reset the camera settings to their default values.

Does anyone have any thoughts on the cause of this problem? Is there anything else I should check before returning this camera?