Is it a derailleur (external gears) or an internal hub like a 3-speed? It's common for well-used derailleurs to start having skips in the high gears, and all it takes is a chain and freewheel replacement.
This is a derailleur, and definitely well abused. It's not a skip, rather a sudden and immediate freewheel. Thinking it's the hub, but the fact that it's not consistently freewheeling is strange. I suppose the obvious thing is to take it apart and look at it rather than write about it on attackpoint!