Evening blue screen of death.

TY. I thought about this since I live in a community. I checked with other neighbors who have the same setup and they have no problem. All cables have been replaced and I even tried a mini box in the living room with the same result. Can't figure it out. It's sad.

Maybe the poor signal is isolated to the living room.  Have you tried the cable box at one of the current mini-box connections?  

I was told the main box has to remain in the same place? By the technician. Is this not true?

I don't know, but I would ask that question when you email cox.help@cox.com.  I'm curious.  Please post back what you find out.

TY. I'll try this in the next 24 hours. Mini-box in the spare bedroom is almost never used, so I'm happy with it in the living room for general programs that I watch with network channels not available with streaming. Streaming in living room is what we use otherwise and works fine. Hope it works. Let you know. 

All cables have been replaced and I even tried a mini box in the living room with the same result.

If a TV setting isn't properly configured, that could explain why the issue occurred in the living room when the big screen TV was connected to either the main cable box or a mini-box.  It wouldn't explain why it only occurs in the evenings though.  Perhaps the OP could test during the evening hours by connecting the main cable box in the living room to a TV other than the big screen.  

Since the issue occurred with a mini-box too, it's probably not because of scheduling.  As far as I know, mini-boxes can't be setup to update at a user specified time and Cox wouldn't schedule updates at 5-5:30pm.

If it's not a TV setting issue, it would seem to be a signal issue.  Since the mini-boxes work in three other locations, but not in the living room, the signal issue must either be 1. to the living room, 2. from the splitter to the cable box/mini-box (if the box is connected to a splitter) or 3. from the box to the TV.  If the OP is able to connect the main cable box (in the evening hours) to a different connection and TV where a mini-box works and the TV gets good audio and video, it will provide additional evidence of a signal issue to the living room.   But, there's still  the matter of identifying exactly where the issue occurs.  The OP didn't indicate if he used the same HDMI cable to the living room TV when connected to the mini-box that was used with the main cable box.  But, a faulty HDMI cable or a bad splitter wouldn't just fail in the evenings, so the issue is most likely caused by a drop off in signal strength due to prime time peak activity.

I wrote the "...TV [being] scheduled to check for updates around this time..." not a cable-box.  You can schedule some makes to check its website for updates.  For 3 years, it could have either been a recurring failed, corrupt or incomplete update.  I understand this sounds extreme and although you chose the word "patience," I would have chosen another.  Resetting the TV to its factory defaults might help.

I don't feel it is an issue with the Cox signal...especially after 3 techs.  The Cox signal would be strongest at the demarc.  The primary feed into the house (living room) would be the closest to the demarc.  I'm not sure where the mini-boxes get signals...either from a splitter at the demarc or in the living room...but I'll assume they're further (cable runs) from the demarc than the main cable-box in the living room.  Again, just an assumption.

Peak network-activity during primetime could be an issue, but this congestion would affect Internet-streaming more than cable TV.  I would assume CATV requires less bandwidth because it's probably more compressed, multiplexed, half-duplex and, at best, formatted as 1080i.  OP reported streaming is okay.

The chronology of a regimented lifestyle could also be an issue.  When do you normally turn on this TV?  Is its coax and splitter near any electrical wiring or device emitting high EMI?  It could just take some time...1 to 30 minutes...for the interference to finally disturb the signal.

Why would you guess 1 type of box would require a stronger signal than a mini-box?  I'm sure the operational specs are documented somewhere, but have you ever compared?  What are the operational specs of a "main box?"

Bruce, I think I read something somewhere that a cable box requires a stronger signal than a mini-box, but I can't find it now, so... I don't know.  I asked Cox and here's what they said:

"The signal strength requirements should be comparable for all devices. The mini-box is a one way only device however so it would not utilize any upstream communications as how other equipment needs a good upstream signal to function properly".

Edit: I think this link may be what I read where a moderator said a cable box requires a stronger signal than a mini-box.  Whether it's accurate or not, I can't say.  In this instance, the cause of the issue turned out to be a defective Contour receiver.  But, I knew I'd read it somewhere.

It makes sense the mini-box would use lower frequencies because it only tunes basic channels.  An "advanced" tuner, however, would need higher frequencies for its features, apps, premiums, etc.  Do shorter waves require more power?  I always thought the longer the wave length, the more the power.  Then again, we're talking about multiplexed signals.  Too bad the OP didn't provide more detail, such as error codes.

However, Error Code RDK-03033 sounds like the OP's symptoms.  Contour is basically X1 and I read on the Xfinity Forum the down signal is fine, but the up signal is out-of-spec.  One fix was an RF amp and another to move a filter closer to the demarc.  It sounds like the OP needs another tech to stop the swaptronics and thoroughly test the up/down signal.