With the case on and no heatsinks the Apollo stopped hashing at high temperatures (40 degrees C ambient).
A restart fixed the issue, but this happened to me a few times in ECO mode with the fan on auto.
I think the fan could not cool the Apollo enough at high temperatures as it is only rated to run up to 40 degrees C ambient.
Have you installed the latest firmware image? You were probably experiencing the MCU overheat issue, and the latest image fixes this issue. If you want to make sure your MCU does not overheat pull it out of its socket and put a heatsink on the ARM processor (the big black chip on the other side of the blue board).
Thats probably the best use of all those heatsinks
The main heatsink/fan for the ASICs is way overbuilt. They can both dissipate over 300 watts of heat at max fan airflow.