2005 Ford Mustang speedometer

Hello, the speedometer starts to gradually drop speed while while the car retains speed, with or without cruise control on. This occurs every time after the car has been driven about 35 to 40 minutes irregardless of speed and it takes about 5 to 8 minutes for it to completely go down below zero. If I stop and shut the car off it works for about the same time again untill its starts failing again. I had it to Ford especially because the speedo is under warranty and they charged me a hundred bucks for a diagnostic and they said they got a code c1236 and pinpointed it to the left rear wheel speed sensor and that part wasnt warranty so I bought the part and did it myself. I also checked for this code with a scanner key on and engine on and never got code. Then I replaced sensor and took it for a ride with the scanner reading live info to me and all sensors were operating with in range and within 40 minutes the speedo did the same thing and there was no change in any sensor reading and no codes while running nor when I shut the car off and rescanned it. I just have to believe its the speedometer but what are your thoughts. Thank you