VTC

Need to check the sensor really, really close. The most minuscule things will mar the image. A light washing over it at an angle will reveal the culprits but only after you attack it from all sides. To clean it only to discover you missed one is common for me. (Had a similar wormy thing on mine before but can't locate the image it presented on.)

Could it be you can't do 4K slow motion, only 120fps in 1920x1080 on the 7iii? That's how it is on the 7Sii. Check camera specs.

What ISO you shooting? Granted I have an elderly 7Sii which is behind tech wise of the 7iii but I rarely shoot above 5000 rolling LOG2 and this is on night shoots. The other thing is cinematographers are always stressing shadow details and realistically that's a bunch of crap. The only ones peering into the shadows are photographers. Pedestrian viewers aren't looking there and if they are then you've got serious problems framing your subject. Face it... When there are two people in the scene interacting the viewer isn't looking past them to learn the brand of crackers that's next to the sink in the poorly lit background. They're drawn to people. To action. To faces. Not to shadows. So back to your problem... Some frame grabs would be helpful.

Make sure there's no fuzz or debris in the viewfinder blocking the send/receive sensors. This seems to be your case other than the sensitivity situation that follows. Another remedy for overly sensitive switching is using LIGHTDIMS or electrical tape. I've done this to my camera. https://www.fredmiranda.com/forum/topic/1394282 https://www.sonyalpharumors.com/sony-tidbits-180/

I have to think it's the junk that MTS files are. If it was a robust and wonderful codec/format everybody'd be shooting MTS, and they aren't. At least in my corner of the universe. Helped a friend edit who did a shoot in MTS and it was painful. Wasn't enough data in the files to get a decent image. Good thing the people paying him didn't know a thing about video other that what they see on their phones. That sample looks anemic, struggling to play smoothly. Hope you figure it out because that look isn't very inspiring.

Get a new mink hair blush brush. That one is too coarse for the sensor. Place the camera in cleaning mode. Sensor is locked stable at end so don't turn off camera until you've completed sensor cleaning. There are times when the rocket blower wouldn't displace the dust and only a wet cleaning with swab would. Got the ever-so-delicate blush brush and haven't needed to wet clean since.

Not dust. Hot / stuck pixels. Dust on sensor is more of a blur or smudge on image. Not as sharply defined as these specks. Return camera. They never self heal themselves miraculously.

For me the USB port for power is useless since I use that port to trigger record or zoom when rigged up. I have a base that uses NP battery connected to a dummy battery in the camera. I cut the provided cord short and hard wired the cable for when it's rig mounted where the power is close to the camera. Quickly disconnected if need be. If I'm running an external monitor/recorder I hook to the base with a different connection I made from a dead battery. If I don't have the camera in a rig I can still use the external power supply by running it off the extension. Sometimes the battery's in my pocket. Sometimes its Velcroed to my wrist.

Not a design flaw but OPERATOR ERROR. Simple as that. Had a 70-300 land on the lens hood straight to the concrete because I didn't rotate it to lock. Didn't blame the camera for a design flaw. Blamed myself for being careless and inattentive. If it was truly a design flaw the internets will be replete with similar horror stories. I suppose Sony could fashion something requiring a key to be inserted and rotated before you swapped lenses but that punishes EVERYONE instead the minuscule number who need parental guidance. Face it... You screwed up. Happens to us all, but only once if we learned something from it.

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Don't know why things would be any different on the 7Riii but on my 7Sii I routinely record with a the USB multiport occupied by either the Sony wireless trigger (in photo) or a Fotga wired remote when rigged up and get video on both the rear screen and a Blackmagic VA 5.

Even if it's not precisely what you're looking for there's still a benefit of using an external recorder. Here's the specs of internal vs. external recording at 1080 on a shoot and a comparison frame of the results. By itself I resigned myself to accept whatever I got internally on most assignments but the colors from the ProRes 442 files did justice to the colors on this one. Though satisfied with the final edit I'm curious what notching it up to ProRes 442 HQ would have turned out.

Odd but taking a wild guess here... Stabilization turned off? It's recommended for tripod use. When shooting with a matte box if I'm not careful shooting towards the sun or have the flag off position I'll get a reflection of the lens on the 2nd surface of the filter but it presents much larger than what's shown here.

A7Sii with shutter stuck closed. Sent in for repair*. Three weeks later it returns. Firmware Ver 3.0 with new settings for timecode and mode dial selected on rear screen from what I can tell but the problem I have is I can no longer trigger to record video or operate zoom on 18-105 lens either with Sony wireless remote (receiver connects to cam vis USB) or the Fogota cabled remote (connects to USB) I affix to the mobile rig. The remote WILL, however, trigger shutter release for photos but is dead for video or zoom. Remotes operate on A7iii fine, just not on my repaired 7Sii. I've gone through the settings and can't see anything I missed that would prevent the operations I mentioned. Any ideas before I send it off under their 90 day repair warranty? *REPAIRS: A complete repair and return all functions to factory. REPLACE: Mech rear grip. Top housing hot shoe. Front grip