Windows 7: Running video disables wireless keyboard

Whenever I run video (youtube, skype, movie), after a short while the wireless keyboard and mouse become disabled. The only way to take control of the computer (it does not crash) is to switch it off, or to plug in a wired mouse and shut down. I have tried:

• changing usb port used for the wireless dongle
• installing new wireless keyboard and mouse (now using Microsoft 6000)
• updating keyboard and mouse drivers

Scouring forums such as these has given me no clue how to proceed. Has anyone any ideas?