All printers will have some fluctuation and it's not a concern. This is similar to the fluctuation you'll see in your home temperature around the thermostat setting. There are a couple reasons. One is that the feedback loop (thermistor to control board to heater to extruder block) will always have some lag. Another is that most systems have built-in "hysteresis," i.e. set the "turn off heat" a couple degrees above the "turn on heat" thresholds. This avoids "chatter" from on to off right at the setpoint.
Some thermistors (e.g. my AnetA8 clone) can be held in place with a setscrew. If yours doesn't have a similar capability, use Kapton tape to hold it in. This tape is designed for high temperature operations so it won't age or collapse, and it has good holding strength.