Check to see if tthe HDMI cable is loose. If not, shutdown the box, reboot the router and modem, do an IPTV Reset at the Donation Checker, wait a few minutes and fire up the box.
Just for giggles...... when that happens again run a speed test. Then run it again. I do that when it acts up here and find that my speed is unstable and all over the place. When my speed is stable I have no problem. Just a thought.