I have been experiencing the same thing but it seems to be prevalent with sports channels. NBCSN, NBC, ESPN, CSN etc. I signed a contract and pay for ALL of these. Comcast needs to uphold their end of the bargain. Ironically there is an additional 5 dollar tax/fee for these sports channels.
Channels are coming in on different frequencies. Possibly whatever is causing your issues it’s only on those frequencies ( maybe others that you don’t watch). A tech can check that frequency(ies) individually onsite, it just can’t be done remotely.