In reply to ungifted_amateur:
I would check:
Chain hasn't stretched,
Cassette isn't worn (caused by not changing chain enough)
Chainrings aren't worn (caused by not changing the chain enough - but rarer than cassette wear)
Remove cassette and freehub body, check the freewheel - clean it, and look for signs of wear and/or sticky pawls cleaning and regreasing (not to heavy a grease) should sort this.
Lastly I would look at dérailleur sprockets... I've had these go and cause issues, but iirc symptoms were general noise and jamming and poor shifting
ETA: If chain is worn to extent it's slipping probably worth changing cassette as well. If cassette is worn then make sure you do the chain as well, if chainrings then most likely you will need to do the lot... worn cassette and/or chainrings will rapidly stretch a new chain.
Post edited at 20:56