Not sure if my experience is the same 'slump' you others are describing but I do get a daily period of feeling decidedly off.
Lasts about 2 hours. Builds up gradually over half an hour or so, stays high for the next hour and gradually subsides again until I'm back to "normal".
It's not related to activity or amount of rest, and also not related to food.
Seems more body clock related, except that sometimes the body clock is slow. There are periods when it happens the same time every day and other periods when each day it strikes an hour or two later than the day before. It rarely hits in the morning though.
There's whole-body exhaustion, leaden limbs and brain fog, plus something weird going on with temperature. If I measure in the mouth my temperature is low-normal - so no fever - but if I measure on the forehead the thermometer does register a fever. My skin, especially my facial skin, is definitely very hot as confirmed by several different thermometers and several people putting their hands on my skin. When it happens at night it wakes me up with night sweats. During the day it's a dry heat.
I get all this with PEM, too, but PEM is more severe and has many more symptoms. So I'm not sure if the daily 'fevering' as I call it is a sort of mini-PEM or something else altogether.
Have no idea how to avoid it. The only thing to do is to lie down and wait it out. I have a suspicion that I'm more susceptible to setting off actual PEM during this 'fevering' period, which I view as all the more reason for a lie-down.