Hi!

I have just changed my chainrings (shimano 105 5703) and chain (KMC X9.99) on my specialized allez elite 2007.
I originally bought this bike used several months ago and it looks like the owner never changed any parts except the chain (all the other components seem to be originals)
When I cleaned the cassette (SRAM 9-speed), it was looking good enough that I didn't change it.

Since I changed the chain and the drivetrain, I experience shifting problems :
- First of all, I have experienced what I think was chain skipping (although i didn't see it, just felt like it)
- Second, when shifting, even after adjusting my derailleur (shimano 105), it has somewhat trouble from going form a cog to another. I don't have a picture, but it looks like the chain has a hard time to "seat" on the cog and therefore goes toward the next cog but before reaching it, goes back and only after jumps to the next cog (I hope it's not too confusing)

It looks to me from this and from what I have read that I should change the cassette.
Could it also be the chain length (too long) ? I am certain I have correctly checked the chain length but I could be wrong