4 cylinder running hot

I've had an intermittent problem with my 2016 Colorado WT with 121,000 miles running hot. It started about a year ago. I was driving a back road that wasn't too hilly or winding at that point, and I noticed that the needle was just touching the red on the gauge. I had the truck towed to a dealership, and they couldn't find anything wrong with it on the next morning.

Between that day and late December, the truck was fine, although sometimes the needle would wiggle a bit. On December 23, I had to drive in a show storm to get to my wife's family's house because our power was out and the temperature was dropping in the house. This is normally a 15-minute drive, but we were driving maybe 20 mph so it took much longer, and my temperature needle started up towards the red again. Sometimes it would drop if I sped up a little. I took it to the dealership again, but they were closed until the 27th. When the reopened, I found out it would be 2-3 weeks before they could look at it. I got an appointment at a dealership about 45 minutes away, so I went to get the truck. It still ran hotter than normal, and then the needle would drop back to around 200 degrees after a bit. I towed the truck to the other shop, and they were also unable to find the problem. They said they pressure tested my coolant system, and there were no leaks. Over the next 2 months or so, it would randomly jump up and then fall back. It seems to be worse when I'm in a drive through lane or sitting in traffic, but it isn't all the time. The needle still wiggles a bit.

On Thursday (2 days ago), I had the spark plugs changed and the coolant replaced. I drove it home yesterday, and I still got random spikes in the temperature, although not quite to the red. I drove it to get lunch today, and it spiked on a slight hill, but dropped just as I told my wife to look at the gauge. I let the truck sit for 2 hours before checking the coolant levels, and there was only a small amount above the hose that comes out of the surge tank. I walked the dogs around the house a couple of times, and went back to the truck about 10 minutes later. The level is still below the fill line, but it had gone up quite a bit. Is it normal to be filling the surge tank over 2 hours after the truck was turned off? Does it look like the dealership didn't burp the coolant system to get it full?

I have heat, and most of the time it happens when the truck isn't being driven hard. First picture shows about 2 hours after driving it. The section picture is about 10 minutes later. Any suggestions about what to try first? I was going to put it on ramps and burp the system before I noticed this. I also thought about getting the temperature sensor changed. I have seen a few posts about the temperature gauge wiggling, but nobody has followed up about what the cause was.