I just did my first bike build up. Everything went rather smooth. The only problem I have run into is the following. When my chain is on the 2nd and the 3rd smallest cogs on the rear, the chain seems to jump if I apply any type of pressure on the pedals, the only time it doesn't happen is on down hills and very flat roads. This holds true for when I am in either the big or small ring.
Is this something as simple as a rear derailleur adjustment, or is it something else. I have adjusted the derailleur at the large and small cog, but was not sure if the adjusters would have any effect on the other cogs. The chain is new, and the rear derailleur has less than 100 miles on it.

Thanks in advance,
Mark