Bicycle Tutor

I recently replaced the chain/ freewheel and on occassion the chain ships. There are no tight links, derailler seems lined up, shifts OK When pulled down the derailler doesn't seem to return to its normal position below the freewheel. I'm suspecting that the chain in not making full contact with the cogs ??? The chainring looks OK but I'm not ruling this out.

If there are no stiff links and the derailleurs are adjusted right, it may be that the chain is slipping up over the top of the teeth under load. This is not too rare after changing a chain because the old chain and cogs may have worn in to each other and the new chain doesn't match up.
Does it do this under heavy load, especially in the smaller cogs in back? If so, you probably need a new freewheel/cassette.