Hotend doesn't maintain temperature
Safety First
Let's look at the graphs. First: you should swap firmware for one that has Thermal Runaway, as, as it is, running about 15 minutes with 28 K less than the printer is ordered to work at is a clear indication that there is no Thermal runaway protection in place - it should have tripped over that long ago! But there is more!
Problem
But this graph and the lack of Thermal Runaway Protection also are typical for printers that have a design flaw: If the airflow from the part cooling fans or the coldend-cooling fan (that's the fan that always runs) brushes over the heater block, it cools it. This limits the achievable temperature.
Luckily, such is easily remedied in one of several ways:

Changing the airducts for ones that does not hit the heater block
Adding a silicone sock around the heater block
Kapton-tape and ceramic wool can be used to make a heater-sock too
Adding an air-shield in the shape of a bit of tinfoil can redirect the airflow away from the heater block, but make positively sure it is mounted Fire-Safe and can't be lost into the print!