I had some experience that might be able to help on this... here's a story:
I had to replace the Folgertech thermistor, which failed pretty early on - less than 10 prints. My replacement thermistor did not have the brass coupling sleeve on it, so I thought I'd try moving it from the old one to the new one. I was able to gently crimp it down to the new thermistor, and for all purposes it appeared to be identical to the Folgertech thermistor.
After this change, filament started to leak out from around the threads of the heat block. This happened even when I tightly I secured the nozzle and throat. I too started messing with the firmware settings and despite my trials, could not make this work correctly. I started measuring the hotend temperature with a laser-assisted infrared thermometer and found it was running much hotter than the thermistor reported. Once I realized the thermistor was not getting a good connection to the heat block, I removed the brass coupling sleeve.
Next, I had a real hard time getting the thermistor to secure down to the heat block properly using just the set screw. I spent far too much time disassembling and reassembling my hotend.
Ultimately, I added silicone-based thermal compound to the thermistor and gently secured it with the set screw. With the paste transferring heat to the thermistor, my leaking and thermal runaway problem stopped. I restored the original FT-5 firmware. This was last week. Since then I have done 6 fairly large prints without any issues.
I used Super Lube heat sink compound. (I know, terrible name...) Link:
I've been using it on both workstations and servers for a solid 5 years now, and it doesn't harden or dry up. It's thermal properties are exceptional, and it's quite inexpensive. The biggest problem is getting it out of the tube without getting a huge gob... but a syringe, gloves and some patience will solve this, making it super easy to apply to the thermistor and hotend receptacle.
I suggest trying something similar. Please post your results, as I am curious to see if this was helpful.