Pre EGR 12.7 smokes and flutters

I have a 12.7 with an intermittent problem. While traveling at slow constant rpm in one of the first four gear. The engine ocasionally will begin to smoke excessively and the engine will flutter. There is no check engine light, and then it just stops and the truck drives fine. It has been happening more and more, usually after I start the engine after sitting a couple hours and then begin driving. I’ve begun to notice that I can tell when it’s about to happen because the engine loses that “crisp” sound. The engine had an out of frame rebuild done about 20k miles ago. Everything except the crank, and cam were replaced