2000 Ford F-150 2000 F-150 shifts hard from 1st to second

Truck shifts hard from 1st to 2nd, when accelerating slowly it is more pronounced, also happens more frequently when truck is warmed up. When I accelerate faster, it won't shift as hard but will seem to "hang" in first gear slightly longer. Just had Ford do an entire tranny flush along with filter change. Another note the hard shift only happens intermitently sometimes hard sometimes almost smooth. The rest of the gears shift smoothly. Can this me a computer problem that can be reset by disconnecting the battery or something internal?