It's possible that the problem was in your fuse box. I run several dozen computers and I've noticed that if a 30 amp fuse starts to overheat, the heat moves to the neighboring fuse and there are power anomalies in my house, clocks keeping bad time, etc. If all your miners are cranking out at peak performance, your house may not be able to deal with the heat in the internal wiring.
I don't think an "overheating" fuse has anything to do with this at all. By design, if the current exceeds what a fuse is designed to allow, the circuit should trip. There's no in between where it somehow can tell your computer to run at half speed.