I have had the black flashes issues in the past, and they seem to continue to be a problem. Unfortunally they seem to be random, but only seem to happen on the HD channels. I have reported them to MTS, but most of the front line support people do not seem to know very much, except how to tell people to "reboot".
In my recent email complaint to MTS about this issue (as well as the PVR HD issue on SD tv's and the requirement to have to reboot every two weeks to keep the system going properly), they replied that "this is not normal and I should phone to get someone to come out and check my system".
I have asked phone support several times in the 4 months that I have had Ultimate to come out and look, and each time I'm just told to "reboot and that should clear it up"