An NTC3950 should work well configured as #1 in MARLIN.
If it is an NTC3950B # 13 may work better.
Be sure to do your PID or MPC tuning.
I would thoroughly check the Amazon page that you ordered it from for the specific settings. (Maybe contact the seller)
Don't forget to use the [LOOKING FOR SPECIFIC INFORMATION SEARCH?] search box at the bottom of the page.
Check the reviews as well.
Be sure the heater cartridge to heat block joint and the thermistor to heat block joints are tight.
While running a print;
Keep an eye on the temperature reading, while gently wiggling any connections between the the heater cartridge/thermistor & the controller. (maybe it's a bad connection)
If you are running an external mosfet, maybe there is a problem with it?
shane
