I realized this is a recurring problem with this machine.
Last time we ran material as big as 1-3/8", the bar feeder was vibrating very badly and the finish on the part was coming out like crap - and it got really bad half-way through the bar.
This material is even worse because it's 1-3/8" with a core diameter of .915 at the root of the acme thread. You can feel it sag when you pick up a full length bar in the middle.
This is an older machine and the feeder channel is pretty beat up and Im told nobody's ever done any real maintenance to the machine.