The 3 speed auto has a VSS ( vehicle speed sensor) that's part of the speedometer cluster. I'm not sure if it's a seperate module or is part of the circuit board on the cluster. It's one of three external sensors that have input to the shifting of the 3 speed auto, and has a hand in controlling the cruise control if so equipped, according to my FSM. The other is the TPS or throttle position sensor, and the vacume modulator, located on the transmission itself. Even though the th180 3 speed is mechanically shifted, those sensors have a say in telling it WHEN to shift. The only other way it'll shift without those sensor inputs is via a govenor according engine rpms. That sounds like what yours is doing, although 5k would seem to be a bit low for a gov shift, usually that occurs at or near redline. You might try checking the vacume line to the modulator. It comes off the intake manifold near the firewall, and goes down to the vacume modulator mounted near the back of the trans pan. If the line's ok, the modulator itself has been known to go on these. Of course I may be wrong about all this, but I've done a lot of research on these trannys. I've got one too.