Something very weird is going on
New Capricorn Bowden tube correctly fitted along with new nozzle.
Also fitted new thermistor (had already ordered them before mnementh suggested Kapton tape)
While I was at it, rechecked all wiring and swapped over the extruder and the X axis cable at both ends to rule out any problem with the extruder cable.
Also fitted silicon springs.
Levelled bed using Cheps bed level gcode, then did 25 point mesh levelling. Remembered to store it.
Ran Cheps bed level test and it came out perfect.
So decided to print a temp tower in case themistor reported slightly different temps. 180 - 220, one I've printed many times before.
Printed this from the SD card in the touch screen port, watched the first few layers and it went very well. So I left it to it. Checked back a while later all was OK. Then it printed the 190 level, then produced nothing but string for 180. My initial thought was the themistor is reporting the wrong temps and the 180 it reported was much lower.
However.
I tried to print again and it couldn't read anything on the SD card. I put the SD card into my PC and it read it correctly. I formatted the SD card and recopied the files over to it. It still couldn't find any files. So I put them on a microsd card and plugged it into the main board.
This worked fine, I tried printing the same temp tower, wondering if the touch screen is playing up (causing problems with SD card), the first section, 220 went down totally fine but this time, when it started the 215 level, string started to come out.
At that point I turned it off and had a beer .
Just gone back, turned it on and now the SD card that wouldn't read in the touch screen earlier, now works totally fine (decent SanDisk Extreme SD)
Wondered if it had lost it's meshleveling at some point, so connected it to octoprint, sent it M503 and sure enough, all mesh levelling details are there.
I just sent it M109 S200 and when up to temp sent it G1 E100 F100 it only fed about 80mm and yet again I can see/hear extruder jumping.
While nozzle at 200, tried to push filament through and it was very very very hard. As though nozzle is blocked again.
When I fitted the bowden tube, I took the entire hot end off and to pieces , triple checking everything. I had the 10mm pneumatic nut firmly secured, the steel bit that goes into the heater block was down just enough so that when the nozzle was tightened right up, there was a 0.5 mm gap between nozzle and heater block (the nozzle was screwed against the steel). Then I removed the nozzle, fed the bowden tube through until it was a fraction past the steel tube (and I pulled on the tube to check it didn't rise back up). I then reassembled everything, bought it to 200 degrees and then put the nozzle on and tightened it. It's almost impossible for there to be another gap there now.
So I've just raised the nozzle temp to 250, filament pushes through easily, even if I feed through filament really fast, zero extruder juddering, I resent it the 100mm test and it was spot on. This is PLA that says 180 - 220 on it.
Going to bed now and will take another look tomorrow, might put original board and screen on and see what happens there.