Diesel Bombers - Doesn't idle like it should

Hello everyone. Here's a timeline of my issues:

-Truck was idling at ~600 in gear. Over the course of a few months it has a tendency to slowly lower itself so I bumped it up to ~750 in gear and called it good.

-One week later: I went on my bi-weekly 175 mile drive home. Lots of highway driving. Came to a stop light and saw it was idling at ~600 in gear, no A/C. Pulled over and bumped it up so I could rest easy for the rest of the drive. Next day I set it at 750 in gear.

-Three days later: (yesterday) Making another long highway trip, I stopped to fill up. Truck was idling at 550-600. It was late and I had places to be so I let it be and finished the trip. I haven't touched it since.

Here's the interesting part.

-Today: Warmed the truck up for around 15 minutes like I always do and proceeded to leave. On my way back from work, which is mostly city driving, it was idling at 750 in gear. There was maybe a slight fluctuation from stop to stop, maybe 50 RPM, but nothing I saw as a concern.

I can't come up with an explanation for this. If the nut on the back of the idle bolt was shaking itself loose, the idle would have dropped and stayed there right? It seems to have something to do with long drives, but I can't think of why. I think if it were the throttle return springs sticking the idle would be more inconsistent than it is. Help me out here guys.