The transmission has shifted to a higher gear causing a drop in rpm. or if its worse case scinereo ie... the rpm goes dead (to 0) and comes back after a bit the cable to your tranny has been stretched
It may just be a normal condition. In auto transmissions with a lock-up torque converter, you will see a slight to moderate drop in RPM when the converter lock-up clutch is engaged. The lower RPMs mean that there is no longer any slip between the engine and wheels. Since you are no longer losing energy in the torque converter (heat created by the viscous resistance of the fluid), you should save a little gas.
Check to see when the drop in RPMs occurs. Is it usually at the same speed? Is it usually in the highest gear/s? The torque converters of some transmissions will do this at lower speeds than others. My Lumina is designed to do this around 38 miles an hour.
When you start to climb a hill, check to see if the RPMs suddenly increase without the transmission actually downshifting. This is when the torque converter clutch disengages and lets the torque converter return to its viscous conversion of energy. This will almost always happen when gradually increasing the load on the transmission before it gets to a point that the transmission has to downshift. A quick increase of the load may cause the clutch to disengage and the transmission to downshift too fast for you to notice it. It will feel as though it is one action.I have a 2000 Pontiac Grand Prix that had the same problem. At any point in time the tach would drop to zero. Then the Traction control light would come on. It would stay that way until I eventually restarted the car. It turned out to be the cam shaft position sensor which caused the problem.