It may be that the paper is too heavy for this printer.
It prints the first page fine because the fuser is fully warmed.
If the paper is too thick, the fuser can't recover temperature in time before the next sheet comes through.
Also, be sure you have "Heavyweight" paper selected on the control panel and not "Plain" or any lighter paper.
I have a custom size heavy paper (9in X 11in). I am trying to print graphics from a custom software. The first page will print correctly, but each of the following pages the graphics pint very very light. IfI try regular paper, it will print correctly on all pages.