Printing with colorfabb XT fails after several successful layers I'm printing parts for a HEVO (HyperCube Evolution CoreXY), using Colorfabb XT filament. After several successful prints. I'm having a lot of failed prints. They start off pretty well but after ~15 layers the filament starts to string and blob. 

Printer details

Model: RepRap i3 
Extruder: E3D 1.75 all metal hotend.
Nozzle: .4
Print temp: 260°C (max. recommended).


Steps I took to troubleshoot


First I thought my nozzle was clogged. But it's not I can push the filament by hand without issues.
Lowered the speed to 35&nbsp;mm/s.
Disabled retraction / disabled part cooling.
increased the max temp with 10&nbsp;% to ensure the filament flow


Printed parts that fail: 


Note: the two parts on the left are printed in one job



UPDATE 26/07
Dit a few more prints with PETG to eliminate the moisture problem. Same results.
This looks a lot like under extrusion caused by heat creep.

Heat creep is when the nozzle temperature 'creeps' up through the filament and makes it melt (a bit) and form a blob (or just widen enough to get stuck) a centimeter or two before the nozzle.

The characteristic is usually that everything works perfectly well for some quite fix amount of time, then there is severe under extrusion.

Solutions:


Lower the temperature (yes, as then the heat won't creep as much!)
Add a fan cooling down the cooling part of the print head


I have the same print head as you it seems (E3D 1.75 all metal) and I had heat creep when I changed the fan to a less noisy one (but also less effective).