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This is a pretty common issue with the 2005 Mustang speedometers. The reason it's acting up is because the motor that controls the gauge is going bad. There's a DIY guide on how to repair it here - 2005 Mustang Speedometer Problem
Also, you can send it in for repair to a speedometer repair specialist.
This is pretty common on the 2005 Mustangs, it's caused by failing stepper motors in the instrument cluster, they control the movement of the gauges. Repair is around $180 and there's a guide on how to fix them yourself here - 2005 Mustang speedometer problem
I have no experience at all with Ford products, but if I had to guess, it would be the that either the connector came loose, or the sensor in the transmission that sends the speed signal to the computer/ speedometer failed. You should be able to see this sensor near the rear of the transmission. It is held in place with a single bolt, and has an electrical connector. Check that the connector is securely in place and that the wires are not frayed or broken. If you find you need a new sensor, they cost about $30.