Drop in temperature after couple layers I'm printing basically a large flat square pane. This goes fine for 3 layers, after which the temperature suddenly drops by about ten degrees. After a while, my printer goes into thermal runaway protection.
I restarted my print, and as soon as the temperature drops, I pause the print. I wait for it to go up again, and resume. It starts dropping again, so I decide to see about continuing this part at 200 degrees (which worked quite fine, so I think I'll stick at 200 for now.)

I have tested running the printer at 210 and turning on the fan, thinking maybe it's simply related to that. That does not give a thermal runaway, and a drop of 2 degrees max. My assumption is that it's the PID.
How do I fix this?
My printer is a heavily modified Anet a8, running Marlin.
I had a similar problem with an Anet A8 once. The problem was, that the thermistor wasn't mounted right in the heat block, so the air flow cooled it while the print head was moving. You can check, whether this is a problem at your printer.