This came up in a conversation with another official.

When a receiver is going downfield he can be hindered as in bumping? True or false? Please cite a rule reference, if possible.

Let me give it a shot....

It depends if the ball is in the air.

Rule 7-8
"Pass interference restrictions on a legal foward pass begin for:
a. A with the snap
b. B when the ball leaves the passer's hand"

Not sure if thats what you were looking for.

Once the receiver is "no longer a potential blocker", he cannot be contacted. 9-2-3d

7-5-10-a "It is forward pass interference if: Any player of A or B who is beyond the neutral zone interferes with an eligible opponent's opportunity to move toward, catch or bat the pass."

7-5-8-a begins for A "with the snap"
7-5-8-b begins for B "when the ball leaves the passer's hand"