Gerbox fault with no codes

This is the situation: the "gearbox fault" sign comes into the display after, say, 4 to 5 miles of city driving (slow city driving), and it stays on and off during the day. BUT if I drive the car to the highway for a while and doing 50-80 mph, the gearbox fault warning goes away. Back in the city, the display iluminates again.

I went to check codes and got a clean reading, no code whatsoever. And no problems with shifting, no limp mode.

Any ideas, please?

go to a jag dealer and get the codes.. read.. perferably when the lights is on