I've been working on a Dell Inspiron 7537 with Windows 8.1 and a Wireless-N 7260 + Bluetooth adapter with the same issue - throughput drops to below 10mbps after ~15 minutes when connected to any wireless network. I can observe other devices connected to the same access point maintaining 54mpbs or 150+ mbps if connected to the 5ghz band.
We are running the current driver and have tried the various recommended power setting changes found throughout this forum. Dell technical support was of no help.
Any help would be appreciated.