Tobyblom

Hi Guys, I have the strangest problem. Installed a GM Performance stock style crate engine in my truck. It ran fantastic for about 1500 miles, then suddenly it starts shifting into 4t gear at 20-25mph. Took it to a transmission shop, had the transmission rebuilt, and it still has the same problem. The shop replaced the ecu, the eprom, but still the same problem. Now it is at a car electric shop, and we cant figure out what is wrong. It gets correct signal from the speed sensor, but when the speedo hits about 20-25mph, the Speed Buffer sends a signal to the transmission that the speed is 70+mph, and it goes into 4th gear. We have changed the buffer, but it still does the same thing. When checking the output from the speedometer, the computer shows that it receives the correct speed compared to what the speedo shows....
Anybody have any ideas? Both me and the car electrician guy are lost here...
Its a 1994 Chevy C1500 2WD, 5.7 engine, 4L60 Transmission