Extruder temperature drop

Hi - hope someone can help with this please?
My extruder loads & unloads z-ultrat perfectly, but when I try to print a model, the hotend temperature initially goes up to about 140 then after around 30% starts to drop - only reaching just above 100 degrees & sits there for ages stuck at about 45%. The bed reaches temp fairly quickly. I’ve replaced the hotend heater and probe (twice) and even invested in a new extruder ribbon cable. It just won’t get past 46%.
Any ideas please???

Hello Dave,

since you have already replaced the hotend, heater&thermocouple and extruder cable - I would go for the extruder PCB since it is responsible for all component plugged into it. As a last resort - the motherboard might be at fault.

Best regards,