On my Prusa i3 my temperature drops from 195c to in its most significant drop of 170c. I have attached a picture of my temperature time graph. What could cause this significant of a drop? Often thermal runaway is tripped as it takes to long to get temperature back up. I am using 1.75mm white hatch box pla through a knock-off j head. 40w heater and standard thermistor. Using Cura for slicing.

Black arrows for marking temprature drops. 1st arrow also marks start of print.

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Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller

Most likely your thermistor is attempting to escape it bonds!

Check it it still securely attached to the hot end. even a mm gap will cause many degrees drop in readings.

I have rebuilt the hotend twice to see if that was the problem. If i leave the hotend idle it stays at the temperature it is set to for hours. I have run PID autotune.

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Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller

Perhaps it is your heater cartridge. The cooling effect is less if you are not running filament through the hot end, just the heat conducting up the heat break. If your heater is not giving its all, then maybe it can maintain temperature sat idle but isn't putting out enough heat when filament is running through.

Try extruding into thin air very slowly, if the temperature drops see if it is less, than when you extrude filament quickly.

Or maybe just replace the heater cartridge anyway.

I am sure there are ways to check the heater cartridge electrically i.e. resistance/current flow that will give you a more precise diagnosis.