As far as I know, the exposure is set the same in viewfinder and live view. i.e., the sensor is not used for exposure. I don't know why they would be different. It could be something blocks the exposure chip when the mirror is down. You have done all the right things, it is probably time to take it in for service.
I bought the camera recently second-handed and I just realized it had this problem. I have tried this with every mode on the camera and it still happens even when set to AUTO. I already did a reset, updated the firmware all to no avail. This is very frustrating. At first I thought I had messed some setting without even knowing about it, but when I tried in AUTO and still got two waaay different pictures through the View Finder and the LCD screen I knew I had a problem. Please help!
What lens are you using?
Try looking at the EXIF data from a handful of images and see which metering mode is in effect for shots that are properly exposed as well some that are overexposed. Is the metering mode changing between Average, Evaluative, Spot Metering, etc.? Are their unexpected variations in the ISO ratings?
As Wadizzle asked, what type and brand of lens are you using?