Windstar 2002 trans problem

Ford Windstar 2002 SE, has transmission cooler.

If more than 5 people in the car, on the highway on the small hills going
up, it turns off control-cruse and displays for few second "check
transmission" message.

I get the same message driving by my self in the city, only when waiting on
the red light with brake pedal pressed on. As soon I move my car the
message is gone.

It doesn't happen every day and cold or hot doesn't matter.

Transmission shop already replaced Neutral Switch (they thought it is a
reason) but it did not help at all.

Twice mechanic was driven with scanner attached but nothing happened; no
message and no code. Computer doesn't show any code.

I checked that as soon as the message is gone (usually after few seconds)
computer doesn't keep any code in memory (except 'check engine' on).

If I have to wait on the red light longer or driving up the hill is longer
the message goes on and off several times.

It doesn't happen every day but it bothers me and I am afraid that my
transmission can be damaged. Changing filter and fluid did not help.

The oxygen sensor light is usually on short after the codes are flashed by
scanner.

Mechanics say that it is because something is wrong with electric or
electronic in my car, or could be that a computer gets wrong signal from the
oxygen sensor.

They suggest do not worry and drive this car safely.

Had any one similar situation?

Does any one know what it is and how to fix it?

How to flash without a scanner a 'check engine' message from the computer
memory (caused probably by oxygen sensor). I like to see if oxygen sensor
has influence on the transmission messages.

The message really bothers me because it is not only a message but also a
laud bip tone.

I was in few garages and no one can help. Ford Dealer screw me before, I
will not go there.

Thanks for the advice.