Is the chain skipping, or the freewheel skipping?

Could be either the chain or the freewheel. If you have a chain wear gauge, check the chain. See if there is a sticky link.

Look to see if one tooth on the freewheel is bent, lube the freewheel.

I would try another chain to see if that's the problem. If it still skips, most likey the freewheel.

Can you duplicate on a stand, or only under load?