'92 Mercury Topaz Speedometer problem

My speedometer on the '92 Mercury Topaz, V-6 engine with three speed automatic. When cold in the morning it will shift thru the gears smoothly. After it warms up, the speedometer speed at 50 - 60 MPH will slowly drop down to 20 to 30 mph and then lower and at the lower point it will down shift to second and then the meter will read 0 mph and it will shift to first and will not shift as I accelerate. Also at this time the mileage meter does not move.......any help would be greatly appreciated...........T :disappoin

Vehicle Speed Sensor replacement usually fixes this problem.

Ireplaced the speed sensor and still it malfunctions. It works fine until it warms up and then as I am traveling at approx 50 mph the speedometer will drop down to 20 and lower....any ideas...I am lost for any ideas........

Is the speedometer making a rattling sound?