j-head thermistor problem

This morning I noticed that the printer did not want to start printing. Looking at the temperature stats (I use Octoprint as host) I noticed that the extruder's thermistor reading was of about 235 degrees (celsius) but the J-head nozzle was warm to touch. So I turned off the printer and checked any short on the breakout board and on the melzi, with no success. So I restarted the printer and noticed that the temperature reading returned normal. but every time I heat the extruder over 150/170 degrees the thermistor reading jump to 230 degrees or sometimes outputs random readings
This is a copy & paste from the Octoprint terminal:

Send: M105 Recv: ok T:142.6 /180.0 B:33.4 /0.0 @:100% 
Send: M105 Recv: ok T:156.8 /180.0 B:33.4 /0.0 @:100% 
Send: M105 Recv: ok T:170.1 /180.0 B:33.5 /0.0 @:0% 
Send: M105 Recv: ok T:175.2 /180.0 B:33.5 /0.0 @:16% 
Send: M105 Recv: ok T:175.1 /180.0 B:33.4 /0.0 @:56% 
Send: M105 Recv: ok T:229.1 /180.0 B:33.5 /0.0 @:0% 
Send: M105 Recv: ok T:237.5 /180.0 B:33.6 /0.0 @:0% 
Send: M105 Recv: ok T:169.9 /180.0 B:33.5 /0.0 @:100% 
Send: M105 Recv: ok T:211.4 /180.0 B:33.5 /0.0 @:0% 
Send: M105 Recv: ok T:184.1 /180.0 B:33.5 /0.0 @:0% 
Send: M105 Recv: ok T:161.1 /180.0 B:33.4 /0.0 @:100%

SOLVED:
the thermistor had a broken leg, I've replaced the thermistor with one spare from the E3D hot end!

If the temperature reads 235 with the thermistor connection broken then it sounds like something is wrong with the calibration.

I had the same problem with my E3D v5 hotend.
The culprit was the ferrules wich are used to make the wire connection with the thermistor.

After a while printing the ferrules snap the thermister wire.
That means the thermistor sometimes still gets a connection and will heat for a while and thermister readings will be faulty.

I believe E3D has tried to solve this issue with their version 6 hotend where they are using round ferrules instead of flat ones.
Still I think metal ferrules are not making the best connection.

I just twist the wires and will cover the entire wire from heater block to PCB with glassfibre wire I had left over from the heatercartridge wich has very long wires.

Thermistors use copper coated steel wires which are not good conductors of heat. That means if you keep them full length you can solder them. There is no need for ferrules. I also solder the heater resistors but it needs to be lead free solder or HMP solder for those.

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[www.hydraraptor.blogspot.com]

My E3D thermistor is soldered too with tin/silver/copper solder and it hasn't given problems.