I had the same problem as you and tried EVERYTHING even had my phone about to get sent back to Samsung repair.
I was searching the forums and came across that you can type the code *#7284#
Type in code *#7284# which will bring you to the phoneutil screen with PDA, Moden, and QualComm USB Setting options.
Click the Qualcomm USB Driver option and select the MTP option a little message will appear @ the button of the screen saying saved, now just press the back button all the out to the normal screen.
Plug in and enjoy :) Did this because for some reason my phone switched out of that regular mode and now it works perfect!!!