Have you manually spun the blades with the blade belt off just to see if there might be a slight wobble? Maybe a wearing spindle or a warp in the deck around its mounting area? Spinning both the center and left blades should provide some insight. Only a timed deck has the blades oriented in a continuous perpendicular fashion. With minute belt slippage they will be perpendicular to its neighbor...and then possibly parallel w/in a short amount of time, especially in thick turf. Just sayin'. My 2-bladed mower will change pitch when the blades are (slip to) parallel - and the cut can then sometimes suffer in tall grass due to turbulence. But with your situation that stripe is always there. Dang. Is there anything dragging under the engine?? Are you using a velke?