Uh, is this a loose thermistor?

It could be a few things. Check all the things.

This is a good start, but if you check all the things and the issue persists, check other things.
- Very true! It's those other things that can sometimes bite you.
Thermistor is the only thing that I can think of... Hopefully tightening it works. It doesn't happen unless the print head is in motion, which makes me think that's the problem. Otherwise, the temperature is stable and completes PID tuning without issue.
- It could be a loose wire, or a one that is about to snap off. If the cable is bad or on the way out, it could easily be screwing with the resistance measurement. I wouldn't think that just a loose thermistor could cause that mess of a graph, but who knows.
  It's the motion bit that you mentioned that gave us just a little more data.
  If you can isolate the wire for the thermistor, jiggle it around at different points to see if you can replicate the rapid temperature changes.

No, thats a graph.

I assume so? The original hot end came with it pre-applied, so I think the spare the printer came with should too, but I haven't checked this one.

That looks like what I've had with bad thermistors or wiring on the edge of totally breaking, surprised it didn't trip any thermal protection, when that happened on my prusa it estopped and my Voron did as well. I def recommend keeping a pile of thermistors with microfit connectors, they're pretty cheap and they can go bad, it's an easy debug step to swap a thermistor out to rule out wiring issues.

It did actually, this screen shot was taken immediately before it shut down. I'll pick up a few replacements and try swapping it out.

No, that's interference from a higher current source.
Most likely your stepper motor wires run in series with the thermistor.

Looking at how the wires are plugged in, you might be on to something here. The extruder stepper seems to be wired into the same port as the breakout board that the thermistor is connected to. If that is the problem though, any ideas how can I fix it and why it's only showing up now? I've had this printer for maybe 2 years now and I installed klipper on it last month, but this issue only began a few days ago.
- Any changes to wiring? connectors?