Transmission problem
2002 Chevy Blazer 6 cyl Two Wheel Drive Automatic 120000 miles

At certain times, when I accelerate from a stop or from a roll, the car will act as though it is in neutral. The engine will rev, but the car does not go anywhere. After a couple of seconds it will grab first gear with a bump and move forward.

Sounds like a trip ,to AAMCO is in your future. Have it diagnosed post results and will advise.