Did the auto-shutoff work before it quited? I mean, did the motor turned off whenever you are NOT spraying? If not, the auto-shutoff is defective and the cause of overheating. I thought that you found out about the 3-way switch and solved your problem from the earlier post. After all, you need to send it back to the dealer. What a bump.
I wonder whether the newer DA400R is inferior to the older model which was made in Taiwan? My DA400 does NOT have a reset switch, but never got overheat either. It is only slightly warm to the touch even after long airbrushing session.