Recently my iMac has been spontaneously rebooting once in a while. After doing some reading, it sounds like this can happen if the system detects one of the thermal sensors getting to high, and shuts down to protect the system. So I started watching the temps while gaming. I've noticed the GPU diode is getting very hot - I've made it hit 90c (heatsink ~68c). However, the fans are all staying around 1000-1100rpm when this occurs. If i manually override the fans to 2k, the system doesn't appear to overheat. But I seem to remember the fans kicking on much earlier before - the point I could hear them while gaming (maybe 2500rpm?) I did do a SMC reset and a PRAM reset, but behavior doesn't seem to have changed. Does anyone know the temperature that the fans should come on? UPDATE: At ~93c diode, I finally get the fans up to 1500, but that seems a bit extreme (Heatsink at 76) NOTE: I did reapply thermal paste to the GPU during a upgrade experiment - its possible the thermal conduction is subpar. Does anyone know what sort of delta-T is normal if the heatsink is at ~65c? EDIT: I did run AHT overnight in extended mode, and everything passed.