Xjyuk

In this story, the narrator describes his day as ordinary objects change their physical properties. The floor might suddenly become fluid beneath one's foot, and you'd better get it out fast before the floor becomes solid again. Food on the plate may turn poisonous without any apparent cause. Mountains march on the horizon as the sky changes color. Life is a series of challenges and emergencies that people take in their stride.

It all changes abruptly one day as everything hardens into an unchanging reality. No more inconveniences, but also no more marching mountains in a golden sky. The narrator can't stand the boredom of it, but no one else seems to notice the change.

Read in English in the 1960s or 1970s, probably in an anthology.

Is it possible this is "The Petrified World" (1968) by Robert Sheckley? It was in his collection Can You Feel Anything When I Do This? (1973).

The protagonist, Lanigan, is from a much more mutable world than ours:

Lanigan looked at his watch. "Six-fifteen." But as he said it, the hour hand jumped convulsively forward. "No, it's five to seven."

He has a conversation with his neighbor Torstein in which we observe normal changes in his world:

"Well, Tom, how's the boy?" Torstein greeted him."

"Fine," Lanigan said, "just fine." He nodded pleasantly and began to walk away under a melting green sky. [...]

"I can't really afford a vacation this month," Lanigan said. (The sky was ochre and pink now-, three pines had withered; an aged oak had turned into a youthful cactus.)

It's Torstein who mentions mountains in a golden sky:

"Trees and lakes," Torstein was rhapsodizing. "The feel of grass growing under your feet, the sight of tall black mountains marching across a golden sky--"

Lanigan ultimately ends up stuck in his nightmare; our "petrified" (immutable) world.

The street at first seemed like any normal city street. There were paving stones, cars, people, buildings, a sky overhead, a sun in the sky. All perfectly normal. Except that nothing was happening.

The pavement never once yielded beneath his feet. Over there was the First National City Bank; it had been here yesterday, which was bad enough; but worse it would be there without fail tomorrow, and the day after that, and the year after that. The First National City Bank (Founded 1892) was grotesquely devoid of possibilities. It would never become a tomb, an airplane, the bones of a prehistoric monster. Sullenly it would remain a building of concrete and steel, madly persisting in its fixity until men with tools came and tediously tore it down.

Lanigan walked through this petrified world, under a blue sky that oozed a sly white around the edges, teasingly promising something that was never delivered. Traffic moved implacably to the right, people crossed at crossings, clocks were within minutes of agreement.

The story was originally published in If, February 1968; you can read it at archive.org.

This sounds a bit like The Men Return by Jack Vance, previously identified as the answer to Short Story -- earth in a pocket of non-causality. This was written in 1957, so it fits your time frame, and it matches in some ways but differs in others.

In the words of the story:

Then came the terrible hour when Earth swam into a pocket of non-causality, and all the ordered tensions of cause-effect dissolved.

The result is randomness just as you describe. For example it has surfaces going liquid then solid again:

He tested the surface of the plain with his foot. The glassy surface (though it likewise seemed a construction of red and gray-green pyramids) accepted his weight, then suddenly sucked at his leg. In a frenzy he tore himself free, jumped back, squatted on the temporarily solid rock.

And food becoming randomly poisonous:

The Relict cared nothing for this; he needed food and out on the plain were plants. They would suffice in lieu of anything better. They grew in the ground, or sometimes on a floating lump of water, or surrounding a core of hard black gas. There were dank black flaps of leaf, clumps of haggard thorn, pale green bulbs, stalks with leaves and contorted flowers. There were no recognizable species, and the Relict had no means of knowing if the leaves and tendrils he had eaten yesterday would poison him today.

At the end of the story Earth leaves the pocket of non-causality:

The shrouded sky was gone; the sun rode proud and bright in a sea of blue. The ground below churned, cracked, heaved, solidified. They felt the obsidian harden under their feet; its color shifted to glossy black. The Earth, the sun, the galaxy, had departed the region of freedom; the other time with its restrictions and logic was once more with them.

The problem is that the story isn't narrated and there is nothing about the narrator feeling bored with the new permanence. I also can't find any reference to marching mountains, in a golden sky or otherwise. Finally the randomness is not something humans take in their stride. Indeed it has reduced humanity (the aforementioned Relicts) to only five survivors. So in these respects the story doesn't match your description.