Odd Issue, Speed Sensor? Trans?

[racdlo2002]

I have a 2007 Silverado Extended CAB z71 4x4 NBS. This does not happen on a regular basis but it has happened 3 times. When i am starting from a stop and i accelerate i notice that my speedometer stays at zero and my rpms go up. I am not sure if the speed initially goes up or if it stays at zero i think it initially goes up. As i am going along though the rpms go up and it is as if the truck is in neutral. I am not sure if this happens because the speedometer is at zero even though i am moving. Once i come to a stop the problem is gone. I did notice that the issue happens when i start from a stop making a sharp right or left turn. Has done it doing both. I am not sure what this is and what I should do. Please HELP!!! Thank you.