Extruder gets colder when the parts cooling fan kicks in, and printer shuts off

[uncut ridge]

from the graph it's clear the temp drifts off after the cooling fans kick in. not sure what to check - do. any help?

[steady sleet]

extruder temp dropping with PCF on is normal to a point (thats why you PID tune at a fan rate you usually use), but not the printer shutting off.

-what hotend do you have?
-what heater cartridge?
-is there a silicon cover over most of the hot end?