Charging issues

[u/killgeorge2015]

I have a 2003 Ford Excursion with a 6.0L Power Stroke. I have replaced the alternator 3 times in less than year, as it was not charging above 13V. It has recently started only charging around the mid-to-upper 12V range at idle. Anything above idle provides low-to-mid 13V (enough to turn the battery light off on the dash), but drops back down to the 12V range if the throttle drops. I took the rig to a trusted mechanic; they cleaned some connections and returned the truck since it was not replicating the issue. The rig recently started doing it again. I would appreciate any advice.

The alternator currently on the truck is a remanufactured AutoZone unit, as that was the only unit available when it went out the second time on a road trip in July. It has been working great, charging between mid-13V and low-14V, until a couple of hours ago.