While building my CNC mini-mill, I purchased a 500W air-cooled spindle from eBay. It came with a power supply, which looked great and I (falsely) assumed it would work perfectly on my home-built mill.
Boy was I wrong! The mill worked great as long as the spindle was turned off. Whenever the spindle was on, there was a chance that the mill’s microcontroller would reset (bad), lock up (worse), or would suddenly enter an infinite loop (really really bad!) and an axis motor would begin moving in some direction until I noticed and stopped it.
After much work identifying and rectifying the causes, the power supply has finally been tamed. Here’s a guide for anyone having the same issues.