Earlier quoted context omitted.
"Anyway, not trying to armchair psych" But I will. If "can't get up to do anything at all" includes loss of appetite, inability to find enjoyment in things you usually enjoy, excessive sleep, and generally depressed mood, I'd suggest you ask your doctor about possible a possible bi-polar II diagnosis. Especially if the "productive" week also coincides with significant challenges to your ability to socialize. (I am no…
Well, I changed my life and felt better :) Now I get 3-4h work done 4-5 days a week.
A week of heavy mental activity that completes the design is a lot of fun! Followed by a few weeks of implementation/bug fix/document is not so much fun, and explains my pattern quite well. Trying to find mentally challenging pieces within the slog, and creating challenges for tasks in the slog helps a lot in the quiet weeks.