My van slip only at 30km?

I have ford freestar 2005 and each time it reach 30km, I feel suddenly boom like transmission slip somewhere and this happen each time.
Thé fluid Is flushed and level ok,
There was recall for torque converter and it was changed also.
Someone say On youtube for this kind of can the reason for slipping is pump shafts.
But I'm not agree because the slip happen only at 30km not before or after.

Get a new one

When an automatic transmission slips it needs rebuilding.

It sounds to me like it's trying to go into two gears at the same time. If so rebuild is in order.

Either low on fluid or it needs a rebuild. Find transmission shop to diagnosis it

Yes

WHAT does it mean by rebuilding transmission?
why it does slip only at 30km? Not before not after