FortiWIFI 60D-POE WIFI service stops when streaming media like youtube

A separate issue to the one I have just posted I am having issues with wifi devices freezing during media playback. To resolve the issue I have to disable and enable the wifi interface on the client device, for example a nexus 4, an iphone 5 a Lenovo x200 etc.

This FortiWIFI is connected to 1Gbit of internet connectivity and I do not have this issue with wired devices.

Is there any settings which drop streaming media or close connections for devices?

Thanks