I have had issues with my i3 Mk3S overheating by 5 degrees or so (never more) ever since I accidentally typed in 1215 as a nozzle temperature to aim for in Octopi. It got up to 290 or so and gave an overheat message.
Since then, when it is heating up, I get this heating graph from octopi.
It's not messing up prints much, although there's more stringing than usual, but is this a sign of impending failure, do you think? And if so, would it be the thermistor or hotend that is the likely candidate?
Can't hurt to do a PID tune. That will run a series of tests to find a good set of parameters for your printer. I wouldn't expect heating it up that one time to be a problem, but there's also no reason not to do a PID tune on occasion.