98 sable transmissions breaking.

I bought my car last april. I was told it had a brand new transmission in it. Around november my car started jumping when starting out. And than it died a couple of days later. I had it replaced and drove it 89 miles and it broke again. Does anyone know what could be causing this problem? The guy at the transmission place is completly baffled.

the Vehicle Speed Sensor may have been the culprit and not the transmission.