Originally Posted by roccobike

I know this is basic, but did you check the interference fit of the derailer? I just had the identical problem with two mountain bikes and both times it was the set screw in the back of the derailer that sits against the back of the frame at the drop out. In both of my instances, that screw was set too low so the derailer jockey wheels were hitting the cassette/freewheel.
If none of the suggestions here work, I'd put the old chain back on the bike to see if the problem remains.
Hope you get it fixed.