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Chain's practically brand new, cassette isn't but is in very good condition. Rear mech second hand but, again, nothing I can see visibly wrong with it (moves from cog to cog fine).
ketsbaia-
Chain's practically brand new, cassette isn't but is in very good condition
New chain and worn cassette will almost certainly be your issue, especially if it's mainly happening in the smaller cogs, otherwise you're looking at worn chainrings, but they look ok so maybe knackered freehub. Cable tension is an indexing issue, so if it's changing between gears look at that, if it's over the cogs but staying in gear you let the last chain get too worn and the teeth have worn to match it and now won't mesh with the new chain properly.
snottyotter
All new parts? What you describe is normally a sign of a worn chain or cogs.