Is it just me, or is every Superman story "Oh man, it looks like Superman's on the ropes! No, wait! He's summoned the energy to win, even though he couldn't do it a second ago!"? It's particularly egregious when he's on the ropes because of Kryptonite. It was my understanding that Kryptonite didn't just make him a little nauseated, it's supposed to make his powers not work.
The worst offender of all time is the latest Superman movie, where Superman is stabbed near fatally on an island of Kryptonite, and only needs to fly off to get some sunlight before he carries the entire island of Kryptonite into space.
Good heroes do things even though it seems impossible. Superman manages this by having his power levels fluctuate wildly for no apparent reason. Maybe there's a rich vein of Superman stories where this doesn't happen, but I don't know of them.