I have a 95 Jimmy 4x4. The speedo is jumpy and will not maintain the current speed. During this period the transmission will not shift properly. After I have driven for a 10 or so miles it will start to show correct speed and shift right. It's my daily driver and need help. I have already changed the VSS and ECM. This jimmy is an OBD2 model and I am unable to download the current ECM or the previous one. The new ECM was programmed by a GMC dealership.