I'm no transmission builder, but I did work in the industry for a lot of years. If it was me, I'd get the thing scanned at a transmission shop you trust. There might be an issue with one or all of the electronic components: the dual pack solenoid (for 3-4 and lockup solenoid), trans pressure sensor, governor pressure solenoid, output speed sensor.
The transmission is a reliable unit and properly looked after can provide many miles of service
