Filament Jamming in Hotend. Why?

[Ramblin' Sid Rumpo]

This has me stumped. I had been printing normally until this happened. Below is an expurgated version of my headache over two days. Some help would be appreciated.

Printer is Hypercube Evolution (CoreXY) using Bowden tube and eSun PLA+ filament. Bowden tube goes from inside the feed cone in the extruder straight through the heatsink and into the feed throat of the heatbreak. Genuine Titan Extruder. Extruder stepper has no label.

Started a new print. Printer was at ambient temperature of about 18 degrees. Printer brought up to temperature, 60 bed and 200 hotend. Bed homed and levelled in gcode. Print speed 60mm/s. Hotend moved to centre of bed and print started. No filament extruded and extruder stepper was making a grinding noise. Normally expect this to be hotend not hot enough. Altered pressure adjustment on the extruder. Made no difference. Cancelled print.
Lowered bed out of the way. Hotend to 225. Attempted to extrude filament. Nothing other than extruder stepper grinding. Tried to retract filament, no motion and extruder stepper still grinding. Disassembled hotend. No problem removing nozzle. The heatbreak and the heatsink Bowden connections would not give. There was filament between the Bowden connector and the heatbreak. When this snapped, I could get the parts free. The heatbreak had stretched filament stuck in the feed throat. There was thickened filament in the Bowden tube preventing retraction. It is still in there and stuck. Extruder stepper replaced and VREF adjusted. New all metal 1.75mm heatbreak. Bowden tube replaced. Hotend reassembled.

Hotend to 200 degrees and extruded 200mm of filament. There was some smoke from the hot end at first and the initial filament was burnt. Everything seemed to be working, hotend turned off and Z-offset calculated and stored. Lowered the bed, hotend back to 200. The problem was back, could not extrude nor retract, extruder stepper grinding. I was able to withdraw the filament manually. The end was slightly thickened with a whispy "tail". Cut the filament, hotend to 225 and re-fed filament. Acrid smoke initially from the hotend and filament extruded. Hotend allowed to cool to room temperature. Hotend to 225. Filament would not extrude nor retract. Hotend turned off and left.

Disassembled hotend. Again, heatbreak and Bowden connections to heatsink would not give. Managed to manually feed filament whilst unscrewing heatbreak. The filament found is shown in the attached picture. The small thick bulge would seem to have occurred in the tiny area where the Bowden tube enters the feed throat of the heatbreak. The thin filament is stretching whilst trying to retract. After that can be seen where the filament has thickened again. Mangled filament trimmed and hotend reassembled making sure that the Bowden tube was seated in the heatbreak feed throat. Hotend to 200 and extruded 200mm filament. Tried a test print, not very good, but worked. Tried a second print, the problem was back again.

Has anybody any ideas how to solve this? I have also checked that the thermistor is reading correctly, changed the roll of filament (just in case I had a bad roll) and have replaced the extruder stepper driver.