Two things come to mind on this, is the shifter adjusted so it is in first only? The second thing is the reaction shaft is loose in the converter from the front pump. My car had that happen last year and again this weekend, I think The car lost a bunch of ET and MPH all of a sudden after stumbling on the line and looseing a bunch of 60 ft
Let us know what you find, I've seen several cars leave like that when the shifter cable wasn't adjusted so the pin on the cable would slide in and out of the shifter arm in first gear, the shifter arm would move a little bit when you pulled the cable end pin out of the arm I've seen the same thing on torqueflytes, there leaving in two gears, not worth a hoot