Answered Says temperature dropped too much when it actually didn't

Why I try to print, I get this message as soon as it hits the x end stop. Error:Extruder switched off. Temperature fell too much during print!

It says temperature fell too much even though the thermistor reads it as the exact same temp I'm trying to print with. I use cura but have gotten this message with other slicers too. I have an e3d v6.

 

When I first was having this problem, I ran this tuner which I forgot the name of to change these kp, ki, and kd values and I tried changing them back to what was default and now it works. At the beginning of the print, the temperature fluctuates from sometimes down to 10 degrees lower, but by the first layer is finished, it becomes stable at the correct temperature never going lower than 1 degree from the print temp.

I plan to buy a 40w heater core to see if that helps. I have the 25w core that came with the e3d v6 kit.

 

I'm having the problem again where it gets the temperature dropped too much error everytime I try to print. Another thing I've noticed is that sometimes when the hot end is heating up it has a lot of trouble once it gets within 5 degrees of the target temp. It doesn't matter what the target temp is, it's always once it's within 5 degrees, it usually cools down a few degrees and goes up and down a bit until it finally and slowly heats up those last 5 degrees. I'd like to try replacing the thermistor and heater core, but I don't know which thermistors from Amazon I can use. It's not that expensive on the e3d website, but it took almost a month for my v6 kit to get here so I'd rather not do that again.

I think this thermistor may work, but I'm not sure.
https://www.amazon.com/Thermistor-R...id=1486520223&sr=1-1&keywords=stud+thermistor.

It says it works with v5 and v6 hotends, but my v6 hotend is the newer one like in this video
.