The writers at DC Comics are well aware of this strange fluctuation in Superman’s power levels and attempt to provide a reasonable explanation in Action Comics #1030.
There is a running joke among DC Comics fans that Superman is only ever as strong as the story needs him to be.
Hopefully, the upcoming Superman movie reboot being developed by J.J. Abrams will bring the character’s cinematic legacy back on track.
This news originated at Screen Rant based on recent DC Comics.