When printing using Octoprint, the nozzle temperature raises during the print at different times. I have to lower the temp setting after I notice the raised temperature. I uploaded the print log to see if I can get help to determine what is going on. I had to lower the temperature a few times during printing, it would go up to 225 when I had it set at 210 in cura for the whole print. I checked my slicer to make sure of all my temp settings and they are where they are supposed to be. I'm not sure what is going on. I'm going to use the SD card without octoprint and see if the same thing happens.
If it were me, I'd review my gcode to see if there are commands in it which change the temperature throughout. Absent those changes in gcode, then I would review the Terminal screen's temperature reporting (and the octoprint.log of course) to look for any changes in the TARGET aspect of the hotend's temperature. If the target is changing then of course the firmware is telling the hotend to change temperatures. If the target is changing in the Terminal (and there aren't any set-temperature-related commands in the gcode) then the next question is why? Then I'd run it in Safe Mode and see if it behaves. If it does, then I'd guess that a third-party plugin is to blame.
Backing up, I think I'd verify that my printer board is plugged into power, it has an adequate power brick, that is plugged into a UPS and all the cabling for the hotend is firmly seated and none of the connectors indicate that arcing has taken place. Of course, if you input power from the wall circuit is dramatically changing then this could present varying amounts of power to your printer board and it could behave badly in some circumstances. I think I'd put a multimeter on the thermistor and see if it's the expected resistance.
Could this be caused by a bad serial cable? It's hard to say. Could this be caused by an undervoltage in your Pi? We don't know because you've not provided the minimum logs as requested.