I had new Douglas Fur flooring installed to match the existing 90 year old D.F. in the adjacent room. Yes they are different colors which is a bit of a problem BUT the real problem was seen after my floor guy stained the floors. There are what I call tiger strips that run back and forth across the floor perpendicular to the way the flooring was laid. They are a couple inches each dark and then light and appear arched like the machine made them as he went back and forth. They look horrible and I know wont disappear after poly. Any ideas? I am assuming it is caused when he sanded or buffed or.. before applying stain. What can be the cause and the cure. New guy? Thanks. Will try to post pics

Can you send a picture?
Sometimes a belt sander can leave a pattern that is caused by the machine's vibration over the sub-floor. It is more common with a single layer of flooring covering the floor joists.
Is your floor the sub-floor and finished floor, also?