Can I make a suggestion? Can you check on your phone for the following setting:
In settings-storage click the 3dot or menu button and go to the USB computer connection....the check mark should be ON for the MTP connection and the PTP connection should be off. (This may be what you referred to in the OP, but thought I would mention it anyways...)
I hope this helps, but you may have checked this already....good luck
Edit: I had to spend a lot of time finding the right drivers for each of the phones in our house...that would work with the win7 64bit machines, but once I got the phones set up properly...it was great. Also, I use airdroid for everything if I don't have a cable, so you might check that one out.