I have the same problem. While driving, all of a

Any warning lights lit on the insturment cluster ? ABS
Vehicle Speed Signal
The vehicle speed signal is generated by the rear anti-lock brake sensor and sent to the 4-wheel anti-lock brake system (4WABS) module. The 4WABS module sends the vehicle speed signal via circuit 679 (GY/BK) to all systems which require a vehicle speed signal input.
Speedometer
The electronic speedometer receives a speed signal from the 4WABS module.

Take it to a qualified repair shop .

Blown fuse or relay

sounds like a faulty gauge cluster or a loose connector on the back of the gauge cluster

Check connectors,pins grimping force,bad contact between pins in main plug connector(abs unit)or connectors near pilar A left.
Good luck!

have obd  scanner run on vehicle 

Most likely it is the ABS module is going bad and needs to be replaced or rebuilt at www.bba-reman.com, hope this helps.

Most likely a speed sensor is going bad (most cars and truck do not use mechanical speedometer cables anymore) most sensors are located near the differential in rwd cars or near the drive shaft on the transmission. Best bet is to get a Haynes manual for your car to help locate the speed sensor. Remove it and take it to the auto parts store so you can visually compare it to the replacement. Install the new speed sensor and it should restore your speedometer and odometer.

on rare ocasions the speedometer gauge cluster goes bad, but usually its the speed sensor. either the sensor it self or the connection either on the speed sensor or at the ecu. sometimes oil gets in and interupts in the electical connections. also make sure everything is fine in the fuse area.

you have a stripped and or binding speedometer cable or gear :)

Hmmm, this is very strange. I have two articles that I have read that come to mind that I believe will help.

It sounds like this happens all of a sudden when driving given that the speedometer and RPM go down, you say, and for this, please review the following article that addresses the possible causes of your engine's abrupt/sudden dying while driving at this link:

http://yourhandymanzone.com/Your_Handyman_Zone_How_To_Pages_Vehicle_Car_Zone_Cars_Trucks_Vans_Engine_System_Engine_Abruptly_Quits_Periodically_While_Driving.htm

Also, since your gauges go down while driving, in addition to the probable causes mentioned in the article linked above, your problem may be a bad alternator according to this article on the sudden loss of a gauge reading while driving:

http://yourhandymanzone.com/Your_Handyman_Zone_How_To_Pages_Vehicle_Car_Zone_Cars_Trucks_Vans_Alternator_Gauges_Go_Up_And_Down_Like_Crazy.htm

These articles should lead you to a solution.