Staying in second gear is "limp mode" which allows you to drive slowly to a repair shop without needing a tow truck. Absolutely do not try to drive at highway speed in limp mode.
You can get an idea of the severity of the problem by when it goes to limp mode. The most serious is when the transmission shifts like normal at first, then, during one of its up-shifts, it suddenly bangs back to second gear. Typically that is caused when the Transmission Computer detects excessive slippage in one of the clutch packs. That can occur during a down-shift too, when slowing to a stop, but that is much less common.
What you're describing is a real common failure for engine sensors on any brand of car, but it can occur to transmission sensors too. That is a sensor failing by becoming heat-sensitive. "Hot soak" is when the heat from the engine migrates over to a sensor and causes it to fail. While driving, natural air flow keeps the sensor cool. Stopping for a short time, as in when stopping for gas, allows the sensor to get hot and fail, then it works again after it cools down, typically for an hour.
The place to start is by having your mechanic read and record the diagnostic fault codes in the Transmission Computer. Those will indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition.
It is important to understand that diagnostic fault codes never say to replace parts or that one is bad. They only indicate the circuit that has the problem. When a part is referenced in a fault code, it is actually the cause of that code only about half of the time. First we have to rule out wiring, connector terminal, and mechanical problems associated with that part. That said, given how this problem is occurring, a failing sensor is much more likely than a wiring problem related to it.
The reason I suspect a sensor as the cause of this is the computer has detected the problem before you start driving. Only electrical / sensor problems are detected right after engine start-up. Mechanical problems, meaning internal transmission wear or parts failure is not detected until that part needs to do its job, and that doesn't occur until the car is moving.
Wednesday, November 30th, 2016 AT 5:48 PM