It's too early to say for sure, but I think I fixed this problem 95%ish with a combination of:
* Stopping coffee, having one only once in a few days. Chronic coffee messes with receptors in gut/brain/who-knows-where-else
* Drinking mineral water instead of tap water
* Drinking alcohol free wheat beer for hydration. (isotonic drink)
* Season now being in autumn, e.g. less dehydration in general
* (I also had the antibiotics treatment, but it did not affect this issue, I'm quite sure about that because as soon as I don't drink enough or take coffee daily, problems come back)
If I sometimes feel this fatigue is coming on, I'll start chug down mineral water, this shakes something up in my gut, then bowel movement, then fine.
Those lifestyle changes above also seemed to fix my "standing intolerance" (Orthostatic intolerance, always self diagnosed, not my doctor).
Over the years, I also often had heightened uric acid levels (with pain in joints/bones/whatever) which I think are just correlated with dehydration (and alcohol consumption, I stopped alcohol in february already).
I restarted SJW (St. John's Wort) because I'm curious about the potential it has for drive/motivation in general and also for POIS)
TODO: I'll link to the relevant threads here in the forum as most of the ideas I got from the forum here
EDIT: Of course magnesium helps too, daily mangesium in low dose should be good..