jskita

I am not even really sure what my set is, maybe 2x8 Shimano Claris set-up, but it shifts and works pretty nicely. Every-so-often the chain will just lift off the cassette and go back down in the same spot

Happens randomly and even just on boring flat straights so there isn't really any sort of exercise I am putting the derailleur through - not shifting at all.

Anyone know what causes this?

Iride01 

Probably either the adjustment is ever so slightly off and it is trying to shift. A 1/4 to 1/2 turn one way on the barrel adjuster might solve it. If it makes it worse, then turn the barrel adjuster twice as much in the other direction. I'd try clockwise first.

Or you might have some issues with your cable housings not being properly in the cable stops or routed poorly if someone replaced them and did their own thing. I once had an issue with my shifter cables sawing into the bb shell and binding up the cable that occasionally had that for one symptom. But it was doing other stuff too.

And if you ever bumped the DR into anything hard or the bike fell on the DR, then the hanger might be bent or twisted slightly.

Bill Kapaun

Turn the barrel adjuster on the RDER a click or 3 CCW. Keep track so you can go back.
Too far and it won't upshift to the smaller cog.

jskita

Great, thank you all for the tips. I thought because it seems to shift fine and no clanking or noise maybe it was something else, but I will try the adjusting.

Bigbus

Check your chain for a sticky link, that's usually what causes it to happen to me where it feels like it's jumping but not changing gears.