The good news is after 35 years or so of putting up with these spells (often, as now, lasting several months) I finally got a diagnosis. The bad news is it's ME, or what we call around here Tapanui Flu.
Oh terrific. They don't know how you get it and they don't know how to fix it. All you can do is learn how to manage it. It seems it's brought on by times of physical exertion and stress. This presents a problem: I hunt in the hills; I ride motorcycles long distances because I like the physical challenge of a 12 hour day in the saddle criss-crossing the Alps; I race a motorcycle against mostly much younger men; I exercise with a rowing machine and weights from 5-7:30 AM 3 days a week; I help run a church and I do this website. It's not exactly like I avoid physical exertion, risk and stress. So managing this is going to require some application.
Today is quite a good day so I'm writing this while I feel OK. I don't know what tomorrow will be like though, but I'll do my best. I was going to write a bit on hyper-Calvinism and the role of God in personal suffering and grief but now that I come to it I haven't got it in me.
Wherever you are I hope you enjoy the sunshine and make the most of the day