Seems like a hardware issue, maybe an incompatibility.
I have no exp with this interface, but you should try to update the firmware file. I also compiled an updated version of hostap that support wifi AC. You can get and try it it from here:
Also check my post abut how to install new hostap and firmware locations:
I am running both interfaces described in the post, in the same time each with 2 SSIDs, one from each in a bridge and one independent and I had no issues for more than a few months.