I've been messing with this thing for way too long. Not finding any help anywhere, especially HP. The issue is this:

Simplex printing is flawless. However, when printing duplex, a dark gray line runs down the middle of side 1. With multiple prints using duplex, the problem worsens and starts affecting the second side as well. I have changed the....

fuser (twice - 1 w/ full maint kit)
toner (three times)
HVPS
duplexer
scanner assembly
DC controller

Paper is standard, printer is squeaky clean. Any suggestions?