Switch the power supply off. Count to 20. Switch it on.
If this doesn't help you might want to try another PSU, try powering one blade at a time, swap controller cables, swap controller ports.
It`s not a PSU issue, i have 2 s5 on 2 different PSU A 750w and a 1000W, and yes they drop to half speed alot ramdomly, ( NOT OC`ed ), sometime after 10-20 mins they will go back up to full speed, sometime not, and sometime totally freeze up and need a PSU reset.
i would have to assume it`s a firmware bug
It might be in your case, but I've had random issues on one unit with "x", "-", blades disappearing, mining sporadically stopping - all went away after replacing the PSU. Worth a try if nothing else helps.