Novakane 

While on the smallest cog of my cassette I've been experiencing some chain skip, and occasionally on the next larger one. The teeth do not seem that worn, the chain does not appear to be overly stretched. The cranks have recently been replaced.
When I was inspecting the cassette I happened to hold the large cog and applied a small amount of pressure to the small one and found that it wiggles (rotationally, not side-to-side). Further inspection revealed that each cog has a bit of play - more towards the smaller, less on the bigger ones. I suspect this is why my chain jumps occasionally when I apply a lot of torque to the cranks. I've never seen this before, and I verified on every other cassette I have that this does not occur.
My question is this: Does this indicate the lock ring needs to be tighter, or has this freehub bit it?

edit: It's a Shimano, btw.

Retro Grouch 

I'm more inclined to blame the cassette than the freehub body. If it was my bike the first thing I would do would be to swap a cassette that I know is OK and see what that does.

Sanchana

Is this 9sp...10sp...?
The new crank with new rings with an old chain and old cassette maybe the problem, depending on how worn.

Novakane 

Sorry, it's a 7 speed cassette and I don't (yet) have the special Shimano splined tool to remove it. To be specific it's one of these: https://harriscyclery.net/itemdetails.cfm?id=724

My concern (beyond the chain skip) is that the freehub is damaged. I've never seen a cassette where the cogs wiggled separately like this - I've not actually changed out a hyperglide cassette before, in case you couldn't tell by the lack of the tool to do so. Is it normal for a damaged cassette to do this?

I guess my worry is that the cogs having a certain level of play at all means that the splines on the outside of the freehub must be worn?

Novakane 

When I examine the cassette further - there's no way this is causing the chain skip. My other concerns still stand though.