99 Windstar trans rough shift

I have a 99 windstar that the first shift it does is rough, but only after you run it about 5 miles. All other shifts are fine including reverse. I have had the trans. flushed out and fluid, filter changed.

..... I have had the trans. flushed out .....

On a machine, by any chance??

A local Garage do it not sure how.

did the rough shift happen previous to the flush? are you confident that the fluid used during the flush and fill is correct for your van?

Yes it was like that. Fluid should be right.

Most "flushes" are fluid exchanges.....which are great.
Some rare situations.....a flush includes chemicals......which is bad.

The correct fluid is a fluid that meets the Mercon V specification.
I prefer to use a fluid that meets this specification.......and not a fluid that does not.....with an additive that is supposed to make it meet the specification.
A good fluid should require no additive.

If the correct fluid is in there (most likely is), then there is something wrong.....and it would be best to have it checked out.
There are a number of "could be" possiblilities.
I would wonder if the fluid pressure were checked......too high can do exactly what you mention......
If the Transmission Range Sensor (TRS)....aka Neutral Safety switch were disconnected.....it would to the same thing......so maybe a problem with the switch?....(you would not be able to start the vehicle if it were unplugged.......just mentioning that something in the switch is used for the shift abruptness).
After 5 minutes could be temperature related........there is a temperature sensor that the PCM uses to adjust the shifts based upon fluid temperature.....
If the cooler is blocked by junk......then the transmission could be overheating....
All these things can be checked out by a competent shop.

It is also possible that a code is stored that indicates a problem that the PCM has seen.......which would lead you to a sensor or other issue.