Chain slipping ?

[Fifelad]

New Cassette, and a chain with plenty of life left on it. Gears are indexing perfectly. I can only describe that randomly the chain “lifts up” for a few seconds, it doesn’t even feel that the chain is slipping off the cogs. It feels momentarily, like the chain is not engaging with the cogs. If the chain and cassette were not compatible it would skip constantly. I’m a bit baffled and looking for suggestions what could be causing this. Thanks in advance.

 

[Alex321]

You say the gears are indexing perfectly, but it sounds like they aren't quite perfect, and the chai is catching against the next bigger sprocket momentarily.

 

[Sharky]

Stiff link?

 

[Ajax Bay]

New cassette = new chain, without the option. If you want to solve this, just do it. If the current "plenty of life left" chain (measures well <0.5% elongated), keep it and put it on when the new chain gets to the same length (per link) and swap them over every 500km.
Assumes cassette and chain are same speed.
Or what @Alex321 said and it's indexing (which the OP has said IS perfect).

 

[azbikes]

Stiff link?

Quick way to look is to put your chain on a neutral gear like middle and middle and then turn your pedals in reverse so the chain goes backwards. Watch it as it goes over the cassette or freewheel and if it jumps that's where your stiff link is. Rotate your chain so it's loose and easy to grab with both hands and then push the chain forward and back forward and back. Then check it again by going up and down. Also look to see if the rivet is sticking out too far in the back/front of the link plate and make sure it's centered.