"If some of the channels on your provider are good, then it's unlikely that the service provider is at fault"
This is not entirely true. Depending on which medium SC uses to carry signals, some decrease in quality can appear without putting the provider (CBS, FOX, whatever) at fault. You have to remember that US networks have no control over how their feed is distributed, while HDnet Canada (which has a license with CRTC) does. If HDnet says it's fiber or nothing, SC will get fiber carriage to their uplink while they can use rabbit ears for US networs, and uncompress and recompress the signal the way they want.
If a channel is bad on a provider and good on another one, it means the first provider is not doing everything it should to avoid losses on the way.