Stalinism lasted until at least his death in 1953. So the period you are describing is all of 20 years, which is essentially a blip on the scale of nations. That same period was also marked by a widespread economic golden age by most of the victors of WWII (France, UK, Japan) and several of the losers (Japan, West Germany), which also ended around the mid-70s, just not as harshly. That’s also around the time communist China started abandoning communist economies for state capitalist ones and started its ascendancy.

So the question is, was communism actually working well in that period, or was it more or less an unsustainable fluke due to the post-war boom era, and as soon as that ended it got left in the dust?

Western Europe, and especially Western Germany, benefited immensely from the Marshall plan. And economic freedom, the early days of what would become the EU and so on. And still, the USSR competed, successfully in the for them relevant fields of sciences and military. And their country didn't collapse doing so. Neither did it collapse immediately after loosing its competitiveness, that took until 1989.

Worth pointing out, Germany outpaced the European allied nations economically pretty quickly, as did Japan.

Being competitive for decades, and the most part of the Cold War, surely is no fluke. In the end, capitalism won out, no doubt about it. Also worth pointing out, capitalism does not lead to freedom, China is a very good example of that.

By the way China, they became the economic power house in the early 2000s. Is the Chinese system a fluke?

"Stable and not dying" isn't exactly a ringing endorsement of a system.

And anyway, they had no freedom, which is the real issue. Communist governance is fundamentally opposed to liberty; the system can't allow it.

I never endorsed it, just pointed out that it worked, it was not the fundamental failure some people want to make it.