When I pedal hard, the chain skips. It only happens when it’s on big chain ring and the second smallest cog combination, but not the smallest cog or any other gear combination. The chain is brand new and the cassette looks pretty new too. Other than that, everything works perfectly, it will shift to all gears without problems. My bike has 50/34 front and 11-36 back.

I find this a bit odd. Please see the video: https://youtube.com/shorts/69ZNXKoyN3w?si=e7DGq4iAcd4jqASv

Does this only happen when the bike's in the trainer, or does it happen on the road too?

On the road too but not when it’s on the repair stand.

You say the cassette looks new, but do you know how many miles it has on it? Did the new chain replace a worn one?

I bought the cassette used along with the sram gx derailleur shown in the video. Not sure how many miles but it looks very new and no worn teeth. I can take some pics of the cassette when I have a chance.

Brand new chain since I installed this cassette.

My guess is that you need a new cassette.

New chain with skipping on one or more of an old cassette's sprockets commonly means teeth on those specific sprockets are worn. The sprockets on cassettes don't wear evenly - some are used more than others and wear sooner. Frankly, that's the only way I personally have figured out that a cassette is worn, if it doesn't work with a new chain in the way you're describing - I'm not good at figuring it out by looking at it - others may be.

The cassette might have well worked fine with the old chain because the wear on those sprockets matched the wear/elongation of the chain that was being used - but you don't know because it wasn't yours. The seller could have honestly said "it shifts fine". But now the worn teeth on those sprockets don't match up with the unworn links of the new chain

Thank you all for your help! I’ll buy another cassette and let you know.

Unior makes a sprocket wear indicator (as does Rohloff for twice the price). They don't seem to be as common as chain wear indicators, I think most people use the "skips under load with a new chain" test. Just keep a new chain and cassette in a drawer - you know you'll need them at some point in the future.

Probably true, but the OP should ride this combo for a couple hundred miles. Odds are good that the new chain and old cassette will mesh better after the miles accumulate, and things will work fine. If not, then a new cassette is the solution. Note that this assumes that the derailleur is adjusted properly and we're not looking at a situation where the derailleur is not properly aligned with the cassette cog.

I tend to do what you described, but I don't think I've given it a couple hundred miles; maybe 50.

Thanks for the pointer. I might get that Unior tool, I have many tools that i use very rarely (if more than once), so this would fit right in! I'm feeling the late winter urge to start spending bike money. I'm also very glad that you and Unior know that it's a sprocket not a cog.

That cog may be favourite gear for previous owner. Only way to tell is to disassemble cogs from cassette, clean really well and take photo of each one from 90-degrees to see how worn it is. It takes just fractions of mm of wear for chain, especially new chain, to skip.

I installed a different cassette, and the issue is resolved. I read all your responses, learned a lot, and truly appreciate your help!