1999 Ford Ranger Losing power on freeway random

The truck runs very good overall, but this last year I've been feeling this stutter or sluggish, almost like a pull in the engine and it's not consistant, some weeks it runs great then the next it might happen 2 times during the week and I've to turn around and go home thinking it was going to break down. I had the fuel pump replaced 8 months ago. I had the transmission serviced, new battery, tires and reg. Oil changes and reg services. Alot of times its first thing in morning, so cold and going up hill. We cant figure it out. I brought it to machanic and he couldn't find anything wrong, he couldn't fix it because it ran great for him. Thanks