The heater cartridge on my CR10 V2 broke so I ordered a new one(12V). After replacing it the new one heats of very quickly past the target temperature and the slowly decreases to the target. Once it hits the target temp it goes up and down by 10 °C or so during prints. During the prints there is under extrusion. I read that if the filament gets hot too high up the hotted it could cause clogs. I replaced the nozzle as well and cleaned out everything.

Some things that I think may have an effect:

1. Could the new heater cartridge not be compatible?
2. does a poor solder job with heater cartridge wires have an effect
3. It's possible I messed up the thermistor when replacing heater cartridge, could that explain whats happening?
4. Is there anything else in the hot end assembly such as fans that would cause this?