Lately my hotend temps are unstable after 10 minutes of printing. Does anybody know how to fix this? Anycubic i3 Mega S (did pid tuning yesterday, did not help...)

Have you checked your connections are all secure?

I had some issues like this that were caused by an intermittent connection on the wires in the hotend area (thermistor).

I would check connections and wiring.

If you have stock heatblock then heating element gets loose. Best solution upgrade heatblock to V6 as it clamps heating element better.

Check wires as others have mentioned and them make sure the heater and thermistor are firmly seated.

If you've never tightened the grub screw on the heater then it's possibly loose.

Question: does that coincide with the time when your cooling part fan starts working and do you have the silicon thermal protection around your heater block?