Hi there!
Just noticed this thread, as i also have a Sapphire 480 nitro OC 4GB.
I don't seem to have the problem you describe in the 1st video but i do experience mouse corruption and i've posted about it
here. It's quite similar to what's depicted in post #3 of this thread.
Never occured to me the problem might be with the card, as i thought it was either the monitor or the mouse.
I have not mentioned this in my topic but i can underclock just fine, until it returns from sleep, @ which point it reloads the default settings due to crash in the radeon ReLive application (latest version). @ 1st, i thought i was being too ambitious with what i was setting in wattman but it still happens when i only change the slight OC of 1305 into 1266, so i stopped doing it.