Maybe it is overheating and blowing it out of the overflow/vent. I think if the front seal was leaking it would leak all the time. You didnt say what model tranny it is but some of the chevys have an overflow/vent on the top. If it gets real hot a little fluid can cover a lot of area especially at highway speed.
Maybe the original leak is still leaking just not as bad. i would go back and look there first.