The reason for asking about the other channels, is that the problem could be a result of the cable company adding new channels that are conflicting with the modulated ones. This is fairly common, and can happen even if the "new" cable channels don't actually have a picture on them, or aren't the same as the modulated channels. It can also affect channels other than those two.

You can test for it by disconnecting either the modulator or the incoming cable feed. If the remaining channels are clear, that's the problem.

The solution is to either move the modulated channels to higher numbers, or add a low-pass filter that will block the higher cable channels from the incoming signal.

Larry's suggestion of (suddenly) low signal-strength is also a possibility...