Did you check the temperature of your hotend while you are printing? Is it constant and is the duty cycle well below 100%?
If the heating cartridge of the hotend is running at its limit, after some time it can't keep up melting the incoming cold filament and the temperature of the hotend starts going down.
If the heating cartridge of the hotend is running at its limit, after some time it can't keep up melting the incoming cold filament and the temperature of the hotend starts going down.