I experienced the same problem 7 months ago, every single month I bought a PSU to replace it up to 3 months 3 PSU. I've been frustrated with that the problems, all the ways I have been done for the fix including advice members like above.
finally, I have found a problem is an electrical voltage in my home not stable (after using a watt meter like Kill A Watt Meter in the USA) because of a cable used not corresponding a procedure or standard for electricity voltage used after the electrician replaced the cable, Mining Rig back to normal and the PSU or USB Riser cable never melt again.
Which cable did electrician replaced ? I want to know this as voltage is not stable in my house too. finally, I have found a problem is an electrical voltage in my home not stable (after using a watt meter like Kill A Watt Meter in the USA) because of a cable used not corresponding a procedure or standard for electricity voltage used after the electrician replaced the cable, Mining Rig back to normal and the PSU or USB Riser cable never melt again.
Maybe every country is different in electrical voltage and cable criteria in used but this is the power voltage and cable criteria in my country.
In a nutshell, my mining room has 4400W electrical current and uses a cable of 2x10 mm2 0.6 / 1 kv then replaced with a cable of 4x6mm2 0.6 / 1 (1,2) kv..
These the pictures:
Before | After | |
 |  |
the point is better use a cable exceeds the load passed to be more secure and anticipate if there is additional power to the front.