Do you think the "flat" spot could just be the slide between gears?
For Example, on an uphill, before I added the Bi-turbo exhaust my Tomos had a "flat spot" around 13-15 mph. when I increased throttle, it made noise, but no power. after I added the bi-turbo, it still hits the 'flat' but at 17-20 instead.
That, at least on my bike, is normal. Cant get enough speed for to shift gears, but is kind of fast for first.
My bike would shift fine on a slight downhill, smooth on level, and a little slow uphill. Like I said, it's normal.