It's actually starting out and staying in second gear. That's "limp mode" and you'll notice it has less power than normal when leaving a stop sign. Internal slippage in one of the clutch packs is a common cause but since the transmission has been rebuilt recently, you also have to consider a sensor or wiring problem.
The Transmission Computer monitors a number of things while you're driving. When it detects a problem, it sets a diagnostic fault code and puts the system into limp mode. Turning the ignition switch off, then restarting the engine takes it out of limp mode. The place to start is by having the fault code(s) read. You'll need to find a mechanic with a scanner that can access that computer. Many auto parts stores will read codes for you for free, but they usually use simple code readers that can only access Engine Computers.
You can get a general idea of the type of code that is set by how the system acts. If the transmission starts out in first gear like normal, then bangs back to second gear, usually during or right after a shift, you can suspect slippage in a clutch pack. If it goes to limp mode while driving at a steady speed, expect to find loss of signal from a sensor due to a failing sensor or a wiring problem associated with it. If it starts out right away in second gear when you start moving for the first time after starting the engine, suspect a wiring or sensor problem that has created an "open circuit". Open circuits will be detected right away when the ignition switch is turned on. That is the same as unplugging the sensor.
Tuesday, September 16th, 2014 AT 12:39 PM