low rpm acceleration stumple

[Rickomatic]

Been a few years since I've even been on the forum, but I've got an issue I'm trying to figure out. Had a rebuilt 6 liter installed a while ago and have about 35,000 miles on it. The past few months I've noticed something that is strange. While on the highway, if I am cruising around 2000 rpm, and start up an incline the engine stumbles. If I give it more gas and it shifts down and the rpm's go above 2000, it runs fine. If I'm cruising at 2000 rpm or below, it runs fine. It is only while trying to slowly accelerate up a hill (whether in cruise control, or with my foot) this happens. I'll note that this happens between about 50 mph and 60 mph. I don't notice it at other speeds.

I've cleaned the MAF and the throttle body, and changed the air filter with no difference. Any ideas on where to look? It is not throwing any codes.