Unstable hot end temperature?

[andrewsi]

From your early pic where it looked like the thermistor is sticking out of the hot end, that could correspond with the 2nd fan blowing on it and making it think it's too cold. Might be worth seeing if there is a way to get it to be more fully inserted.

[Bachgassenbande]

Try to turn the fan off and see what happens then, no need to push in the thermistor more to verify

[Dreide]

Meanwhile I worked on my hot end. I pulled out the thermistor and measured the hole, which turned out to be just about 5mm deep, so I decided to drill it all the way through. I put the thermistor back in and sealed it with high temp silicone. Now the temp curve looks like this:

... at a different temp scale:

The snapshot was taken during a print of a rather big part and begins at layer 10 or so. The 2nd fan was running at different speeds for the perimeter and the infill. I noticed that the temperature noise level goes somewhat up at higher layers which I think is caused by the cooler air up there due to the increased distance to the heated bed.
So, problem solved I guess.