Signs of a failing Thermistor?

Hi, I'm printing now for a few weeks with the G3D Budaschnozzle and I've had good results.

Now since a few days I have some weird effect in my Repetier Host, the temperature goes up less quick than before and eventually when it gets around 210º, suddenly in a split second it jumps to 189º and back to 233º, not in a linear fashion, it just displays those three values withint a few seconds.

Since a temperature can't possibly fluctuate that fast, I guess there must be something else wrong. It's either the firmware interpreting the values weird, or it is the thermistor beginning to fail.

When my hotend reaches target temperature: 227º, it doesn't fluctuate that much anymore, although, since the fluctuations have started, the temp sometimes drops to 204º and then quickly climbs back up. I don't know where to look!

I am thinking, it can be: thermistor, ceramic heating element, the mosfet that supplies power to the hotend or wiring. I have checked wiring and haven't done anything funky to it.

Running a Sanguino with Marlin.

I can videotape the fluctuations, but I think it's clear already :)