How do you determine what is causing a 1989 Cavalier to stall when put in Drive?

I had a similar issue with my 1994 cavalier, it ran well during short trios, but after she was warmed up, and went a distance, she would stall out at red lights and in slow moving traffic. We found out that my torque converter locked up and now that problem has cost my a new tranny, because it started slipping, becasue of the torque converter. Now after fixing the tranny, we can't get it to turn over, it has no fire to start.

i had a miracle mechanic took him 3 min. to fix the problem after 5 different mechanics tried . all he did was to disconnect the transmission sensor switch and the car hasn't giving any problems ever.