1999 Chevy Suburban Hesitation

My Suburban is giving me a some trouble. When starting out down the road everything is great until I hit about 40-45 mph then it starts bucking, or hesitating. This lasts until I hit about 55 mph. If I gun it it won't hesitate. Once I am past 55 it runs out fine. Seems to only be an issue when applying the throttle gradually. I have replaced the plugs and wires, just replaced the starter (was told this issue could be starter related), and have run Berryman B-12 through the fuel system. I am at a loss of what to try next. Sure could use some help.

Thank you