2005 Vauxhall Corsa easytronic gearbox is not switching from 2nd to 3rd gear, flashing f

Hi
I have a corsa 55 reg 1.2L, easytronic gearbox. A couple of weeks ago it started flashing F and service indicator lights up and stays on. Car starts and can be driven but it does not switch gears from 2nd to 3rd and cannot be switched to manual option. Break lights are out, only the middle one lights up. It does not start flashing F when it has just been started, only when its driven. I also noticed it often starts doing it when i sharply turn left. Stopping and switching engine off doesnt help, however putting it in neutral and rolling works at times.
Had it looked at and it came up with faulty break light switch code and other codes. Codes were wiped and car driven until it came up with F again and the only code that came up was the faulty break light switch. Switch was changed and drivers side break light bulb too. Its run ok for couple of days and then the same problem occured again:starts, can be driven but flashing F and does not switch between 2nd and 3rd gear, break lights out, the middle one comes on.
Does anybody know what else it could be? Thanks for your help.