Gearbox fault light/loosing speed

hi I've had my car around 5 years this has happened quite a few times over the years..i start the car gearbox fault mesg come up on dashboard..i put the gear into reverse but doesn't move I try few times put gear into neutral then reverse and car starts to move and light gearbox light goes off I have checked by the garage and they cant find the fault. Recently whilst driving speed drops and I have to accelerate to speed up very annoying especially as I set speed to 30miles. also jutters whilst on motorway I feel I have to hold steering wheel tight to keep control.