This is a very late response but I hope it will be of some use for the original poster (jonnyboy) and others.
During my first couple of years with this illness I had hardly any cognitive problems. My problems were all physical - dizziness, nausea, increased heart rate, trouble with heat, muscle twitching, exhaustion after very little exertion, and so on. It took 5 years before I got a diagnosis of Orthostatic Intolerance (NMH in my case, not POTS). I may have had some problems with foggy thinking and forgetfulness but I assumed it was because I was so exhausted.
When I partially recovered, and was able to return to work (software engineer at a large firm), I was able to write software, write specifications, attend meetings, and do all my other tasks without much problem. I would come home and collapse, and I'd spend the whole weekend resting, but as far as I could tell it was exhaustion due to physical exertion and not from mental work. And then later I realized that part of the problem was the NMH and that even just sitting upright at a desk for long periods can be exhausting (need to have my feet up). However, if you had asked me then whether I had any cognitive problems I would have said, "No, absolutely not." Then gradually I began to notice changes.
First, I had trouble with nouns. But not with every noun, just with concrete items. I never had any problem with words like algorithm or schedule. I used to joke that while I was having aphasia I never had trouble with the word aphasia. (I learned later that's not quite the right term, it was more anomia)
Anyway, I would say things like, "Hand me the, um, um, that thing on the table, square, black, has data on it ... DISK! Yeah, please hand me the disk." If it was some concrete item that I could picture in my mind then it might get turned into a guessing game in mid-sentence. I would be absolutely sure that I knew what I was going to say and suddenly my mind would go blank. Fortunately, this type of thing happened infrequently, and it was easy to pass off as a joke, so it didn't interfere with my work.
Second, I had trouble staying on task. If I got interrupted I would completely forget what I was doing. This was a completely new experience because I was always very good at being focused. After I finally figured out this was a new problem I used to joke that I suddenly had a stack depth of one, really more of a register than a stack (jokes for my fellow computer nerds). I learned pretty quickly that I needed to keep a written "to do" list with me at all times so I'd know what I was doing in case of interruptions (e.g., phone calls). Fortunately, I worked in a small cubicle where there was generally some visual clue (either my computer screen or my "to do" list) to jog my memory.
Third, I had a lot more trouble learning new things. I could easily write program in languages that I had already learned before I got sick. But learning new languages was suddenly a lot harder. I was only 29 when I first got sick, and in my early 30s when I first noticed this problem, so this was clearly not an age issue.
Now, after 20+ years of illness, I have a lot more cognitive problems than when I first got sick. However, I'm nowhere near as bad as people who say that they can't even think well enough to read a book. My cognitive problems are worse when standing, or during a "crash." My problems can be reduced if I manage to get complete rest for a few days in a row and then get a really good night of sleep (very rare).
I also wonder at times whether all my problems could be due to my NMH. I do fit the CCC criteria for ME/CFS (and I think the ICC criteria, too) so I guess that's not true. And I definitely have PEM/PENE (or whatever you want to call it). But the whole situation is still a bit confusing for me.
Well, there's one more data point for you. I'm not sure whether it completely answers the question but I hope it was helpful! If I left out something feel free to ask.
