I have exactly the same issue using PLA and standard settings. As the print layers increase the nozzle temperature, as shown on my CR10 display- ( both the Set temp and actual temp ), slowly drops, layer by layer. Ultimately, the filament becomes too viscous and the extruder starts to snatch back as it can't push the PLA through the nozzle. I can override the setting briefly, raising the nozzle temperature, then the extruding is fine. The next layer change, the Gcode calls the lower nozzle temperature again.
;ChangeAtZ V5.1.1: executed at Layer 50
M117 Printing... ch@L 50
M104 S196.118012 T0
;ChangeAtZ V5.1.1: executed at Layer 50
M117 Printing... ch@L 50
M104 S196.753247 T0
;ChangeAtZ V5.1.1: executed at Layer 50
M117 Printing... ch@L 50
M104 S200.000000 T0
;ChangeAtZ V5.1.1: executed at Layer 50
M117 Printing... ch@L 50
M104 S204.807692 T0
G0 F9000 X190.389 Y61.242
........................................................
;ChangeAtZ V5.1.1: executed at Layer 150
M117 Printing... ch@L 150
M104 S188.354037 T0
;ChangeAtZ V5.1.1: executed at Layer 150
M117 Printing... ch@L 150
M104 S190.259740 T0
G0 F9000 X116.789 Y80.745
...........................................................
;ChangeAtZ V5.1.1: executed at Layer 250
M117 Printing... ch@L 250
M104 S180.590062 T0
G0 F9000 X207.686 Y59.878
G0 X207.94 Y60.929
I've searched high and low to find a solution to this......
Thanks.... you all do great team work......