Simsalas

From time to time, I have no speed signal on my 1988 Pontiac Trans Am GTA with digital dash. When the car is cold, the speedometer shows the correct speed. After a few miles, when the car is warm, the speedometer on the digital dash temporary shows "100" instead the correct speed.

Whats the problem? Can it be a broken speed sensor on the transmission or something similar? Whats the part number on Rockauto or Summit Racing?

Thank you.