Thanks 57. The Videotron technician has already been to my house to change the splitter (I only have one) and connectors. The signal didn't improve at all on the affected channels, and is great on all the other channels (as before). My broadband Internet is also working fine. The technician was stumped and couldn't figure it out. That's why I suspect it's not really a signal issue, but perhaps a software issue with the 8300HD not tuning to the correct frequency to receive the QAM (which would explain why the signal level is low, if the box is tuning to a frequency with no QAM on it). This is just my hypothesis, but I don't have any evidence.

-cinergi