Gearbox problem after 120kp/h

Hello!

I might have a little problem with my car. I have a Smart 451, and 2 weeks ago I went on the highway. When I was close to hitting 120 km/h, the 'A' on the dashboard was flashing, and also the wrench icon. I tried to put it in manual, and the 4th gear indicator was flashing as well and refused to go into 5th. I had to stop (the 'N' gear indicator was flashing) and wait for a while, and then start the engine. After that everything was okay; I've never had a problem since then (but I was too afraid to go near 120 km/h again).

I went to my mechanic and he told me that the problem is the Gearbox Controller.

My question is... can I do something to fix it, or do I have to look for a new one?