Chad Hicks

Also - The speedo works until the issue occurs

Hi all, I have a 1991 Chevy Silverado 2500 2wd that is acting up a little bit. It will drive and shift totally fine for about 8-10 miles, then it will lose the speedo and downshift into 1st gear and stay there. If I park it and wait until the engine cools off, it'll work fine until it warms up again. The issue never occurs until I've run the truck for about 8-10 miles. The cruise control does not work either, all fuses look ok. I knew I had a leak at the transmission gasket so I went ahead and got the transmission serviced (new filter, gasket, etc), but of course this issue remains. Checked the OBD code and its giving a 24, but the truck isn't moving so it says to check the connection between the ECM and the VSS. I guess the part I am not understanding is how it could be an electrical connection issue that only occurs when the engine and transmission is warmed up, and goes away once its cooled off? I don't really want to send a mechanic on a fishing expedition, so if anyone has some ideas for things I could check before getting a shop involved, they would be greatly appreciated. Thanks in advance!