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Background: New chain, new bottom bracket, new front derailleur, new shifters, new brakes.

This is a cheap Walmart Mongoose that I'm trying to keep riding and upgraded some of the components recently, but now I have a clang sound that is simultaneous with a pedal rotation and what feels like the chain slipping. When I pedal with any force there is a hesitation then drop from the 3 o'clock to 6 o'clock rotation of the pedal.

It happens more often if I'm going uphill. I'm not riding it; this is just me testing it on my street and driveway which has an upward slope. It feels like I'm going to break something, either the chain, the pedal or myself.

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If you replaced the chain without replacing the freewheel (if it's a Walmart bike then I'm assuming you have a freewheel, not a cassette), then chain skipping is the classic symptom of a worn freewheel/cassette. This is because the chain and freewheel wear together. The new unstretched chain you put on does not mesh with the worn freewheel and you get skipping under pressure.

So the solution is to replace the freewheel.

This other post explains in a bit more detail

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  • "The new unstretched chain you put on does not mesh with the worn freewheel" - I've heard this theory before, but I'm pretty sure it's wrong. What is true is that a new cassette only meshes with an unstretched chain. As the cassette wears out, each gap between the tooth becomes wider, which means a) it becomes more "forgiving" WRT chain length b) the chain is held less firmly by the teeth. As a consequence, a worn cassette can mesh with a stretched chain, but it can also still mesh just fine with a new chain. Wear does not change the distance between (the centers of) the teeth.
    – leftaroundabout
    Commented Oct 31 at 16:39
  • ...What this means for practical purposes: it is certainly correct that one should always put on a new chain when replacing the cassette, but OTOH it is perfectly reasonable to put a new chain on an old cassette. Only, at some point the cassette is so worn that any chain will skip, and then one needs to replace both cassette and chain.
    – leftaroundabout
    Commented Oct 31 at 16:43
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I ran into this recently after I degreased and re-assembled a cassette - one of the cogs was doing this on every revolution. I thought for sure it was a worn cog, but when I disassembled it, I noticed that the spacer for the problem cog was a bit narrower than the others, I had accidentally swapped it with the spacer that goes between the biggest cog and the hub.

But if you haven't touched the cassette, I'd bet on a worn cassette/freewheel as described in the other answer.

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