My transmission will slip shifting from first to second gear

When I take off after sitting still for a while, my transmission will slip shifting from first to second gear. When it does this, I can move the gear selector out of drive into 3, 2, or 1, making it drop back into first. After a few seconds in first gear, it will shift to second fine, then to the rest of the gears, like it is supposed to. After this first slip, it will shift through all the gears fine, unless I stop somewhere for a while. I have changed the filter and fluid and the problem is still there, not quite as bad.