UPDATE
Had my gearbox checked out last week and garage could find nothing wrong with it even after running a diagnostic and road test. They suggested I may have an electrical fault that has been causing problems. When I had the problem on the motorway a couple of weeks ago, the car boot was jam packed and I had to force it shut. Turns out that in the boot lid I had some frayed wires that have been causing a few problems and may have been causing error codes as well as causing rear number plates to flicker. I have just had wiring in the boot fixed, so hopefully this solves the problem.