I assume you "removed" the stiff link after chain assembly?

It could be your chainrings. They wear as well, albeit much more slowly than cassette cogs (they're larger, and it's usually the small cassette cogs that show symptoms first). But I had chain skip due to a worn small ring many years ago. It took very high pedal loads to induce, but sure enough, "BAM!" it would skip.

Cassette pawls could also be the culprit as mentioned by ramzilla. Thing is, pawl issues usually manifest during all pedaling situations, not just under high loads.*

Bent or broken axle COULD be the culprit as well, but very unlikely with a cassette. What could also be the issue would be a loose freehub body on the hub. Any flex under load in the interface between hubshell and freehub body would cause problems.

Another possible cause would be your rear derailleur. Derailleur tight in hanger? Is the hanger straight? Pulley wheel bolts tight? How's the cable housing? It ages over time and develops friction. If this is the case, frame flex during high pedaling loads can "break" the high-friction cable loose and cause an effective change in derailleur position.

Which comes to frame flex... If there's enough flex, the geometry of the cable between you shifter and derailleur can move enough to "change the gears."

So there are many potential causes other than the typical "new chain on a worn cassette" skipping problem.

*I say this in reference to single bikes. Not tandems. They're an ENTIRELY different beast. High pedaling loads on mountain tandems are BRUTAL on rear cassette hubs. I've destroyed six hubs so far, the most memorable being a Phil Wood FSC splitting in two on the Slicrock Trail!