I just replaced my cassette, do I need replace the 2 month old chain? I replaced both my cassette and chain in July (2.5 months ago).

For some reason my chain started skipping when I was in the smallest cog in back. I took it to a shop, they could not find anything wrong. They measured the chain and the cogs and said everything looked good, but since it was not working they suggested I replace the cassette and chain again. 

So, I bought another cassette and another chain. I just put the cassette on and the bike works great. 

Should I put on the new chain too? Or save it for a while? 

Thanks,
If it works great, leave it alone. Measure the chain every few hundred miles and install the new one as soon as the one you have on now is worn beyond spec.

And, when you have a few minutes to spare, read this: http://sheldonbrown.com/chains.htm