When hot outside temperature gauge starts to be erratic. It jumps up with acceleration, or going uphill. Needle reaches top and then it drops down, even if going uphil. It happens only when hot outside and after some driving time. I also expecting longer heat up after start, but hard to judge in summer... Is it stucked thermostat or?
Video: https://www.youtube.com/watch?v=ihUxoJUTzes

Can you read the engine temperature with a diagnostic device?
It's possible that your engine temperature to be about 113-118 degrees in those moments...

Are you using AC?
When stationary and idle, with AC on, the engine fans are working?
If they are working, clean the radiators, especially between them.
If they aren't working, solve the problem ASAP.

Fans working. They cool down even after engine switch off. Radiator clear. No ODB tool at hand

If they cool down, even after engine working, then the engine is too hot, like I've said.

Maybe radiator looks clean, but like I've said - between radiators could be a lot of dirt.

Or, indeed, there is a problem with the thermostat...

Did you tried to feel the radiator hoses? If they are too hot, then dirt between radiators. If they are only warm, thermostat.