See, they had to go "Crisis" with a 'K' or it's a whole thing...

...also, it's "krimson" Kryptonite, because red Kryptonite would be hokey, right? Title hijinks aside, this is still a pretty good one: from 1990, Superman #50, part four of "Krisis of the Krimson Kryptonite," "The Human Factor" Written by Jerry Ordway, art by Dan Jurgens, Brett Breeding, Kerry Gammill, Dennis Janke, Curt Swan, John Byrne, and Jerry Ordway.

After getting his ass thrown out of the Lexcorp building, a glum and powerless Clark Kent recaps the recent storyline: Lex Luthor had used a mysterious red rock, to take away his powers and give him a solid thrashing. Swiped with the help of the Will Payton Starman, the rock was now being studied by Professor Hamilton, who found it wasn't anything like Kryptonite; but that wasn't helping yet. Forced to ride the subway, Clark's there when a giant mutated rat attacks his car, and still tries to help a downed man, putting himself in harm's way: he's luckily saved by Guardian. Later, on the streets, he sees a mugging thwarted by Gangbuster: Metropolis's other heroes were trying to rally to cover for the MIA Superman.

Back at his apartment, Clark gets a call from his mom, who left a surprise for him in his suitcase: a wedding ring. Y'know, just in case he might need it or something. After some drama at the Planet, Clark makes a somewhat awkward proposal to Lois at lunch, but she has to think about it. No time now, since she's got a call (on the restaurant's cordless phone!) from Lex Luthor! He offers to help Lois's ailing mom, if she will become his official biographer. Lois says no way, but Clark grabs the phone, to demand a meeting. He tells Lois this wasn't news, but to help Superman. Clark meets Lex, who threatens charges against him and Starman for stealing the rock, but Clark just wants the story, off the record. Dying of Kryptonite poisoning, Lex was desperate to bring down Superman...but, by "rule" couldn't tell him how he did it. Still, if he told Clark, that wouldn't be telling Superman, right? And he had to rub both of their noses in it: the red rock was a gift from "the imp from the Fifth Dimension...Mister Mxyzptlk!" Cue one of the best page turn/cutaways ever in comics, as a purple-and-green Mxy is very obviously hassling the Fantastic Four! Circa FF #11, and I always forget the Impossible Man appeared that early.

Changing back to his DC look, Mxyzptlk storms back to give Lex an earful, as Clark starts to put it together, since his powers were returning. Lex and Mxy are arguing about it when Superman shows up: Mxy had irons on the fire in other dimensions, and thought he could bug Supes by "remote control" with the rock. Now mad at Lex, Mxy tells Superman he'll leave for the usual 90 days, if Supes will punch out Lex. Superman refuses, but since he wants to get back to "that other, fun, world" (careful!) Mxy "helps" him by cloning a giant blobby monster from Lex's skin cells, so when Supes punches it, he was free to go. Back on "fun world" Mxy changes his color scheme and is ready to play, but the FF has copped to the fact they can ignore him and he'll go away.

After a bit more subplot work, Clark catches up with Lois at the hospital, where her mom is improving, and she has her answer: yes! They share a romantic kiss, in hospital parking, which I've cut off there...Keep in mind, there was still a few issues before she would be let in on his secret identity; but I think Clark felt he had to win Lois over, not Superman. 

I would've been buying Superman comics casually at that point, post-Byrne (and I wasn't all-in with him, either) but I would've bought the crossover issue, Starman #28! Along with the "You will not believe the last page!" cover blurb, Swan and Byrne on the cover would also have been a draw; but this was pre-Twitter, pre-comics internet, so the FF cameos were a complete surprise! That was neat, although some readers have since twisted themselves in knots trying to get all that to fit into continuity: it's just a laugh, alright? The only thing I could think of that might have come close--it's not as good, but would've fit in 1990--would've been if Mxyzptlk had to hustle back to the 24th century...