Don't know if this will help..

https://www.whatcar.com/news/ford-acts-on-speedometer-faults/n3816

Ford charged my dad then after the warranty expired refunded it after this came out but it was a long time ago.

sounds like you have an electrical issue.. speed testyou might want to perform a voltage drop test with a digital volt meter first..

you could also have the electrical portion of the ignition switch failing. that can also be tested by monitoring the varioushowboxs circuits as you slowly rotate the ignition key and probe the larger wires in the connector..

don't change the alternator until you have proven its bad.. could be really simple. and alternators only recharge the battery and supply voltage to the car.. it will not effect individual circuits like you describe..usps tracking