Yes the stock heatsink is not designed for fanless operation. Even at base frequency and lowest voltage you risk overheating with no airflow.
Running 10 ML2 on Sipolar A400. Had an Arctic USB Fan blowing on them on the side. Ran into random ASIC stop hashing (likely due to overheating). Your driver update brought them back to life, but would die again shortly. Anyway, bought a bigger $4 USB fan from Daiso. That actually moves more air and keeps the "stopped hashing" error message away. So, I believe it was a heat issue.
Daiso Fan here:
https://i.imgur.com/GsVRLs1.jpgYeah my conclusion is remove stock fan and use a good fan.
my sticks are on 756 freq and do 5mh
they are not hot.
eventually I will run eight with the scythe fans.