Most likely one of 3 things :
1. Transmission fluid level low.
2. The conductor plate in the transmission is going bad.
3. bad TCM ( we hope not - $$$$$$ ).

Conductor plate ( I am not affiliated with site ) :
http://www.ebay.com/itm/Transmission-Conductor-Plate-for-Crossfire-Sprinter-Van-Grand-Cherokee-MB-NEW-/370837813895?pt=Motors_Car_Truck_Parts_Accessories&hash=item5657a7fa87&vxp=mtr