When did you change transmission fluid & filter last? If it has been more than 40K than it should be done as part of maintenance plan.
When pan is off, have a look for debris in bottom of pan. If none - this is a good sign.
Many transission shops will provide free assessment, even without doing the fluid & filter change.
GMs did have a problem with shift to neutral under heavy trottle problem caused by shift solenoids and updated solenoid design twice before 1996 model year. However, most of the time this problem ocurred when at highway speeds, not from a stop.
I would try the easy route first with transmission fluid & filter change as this often cure a lot of transmission quirks.