Automatic gearbox??

Lets see if anyone can help me here??,I have had a problem before with this,i turn car over was stuck in 1st gear no p/d/r/n come on the dash only the mileage,took to garage they fixed with putting a code in,great,its lasted a year now gone again,took to garage they got it stuck in 3rd,got it home but now its out 3rd but on turning the car on i leave it for about 5-10 min and low and behold the p/d/r/n all come on and its been fine to drive.i am now pulling my hair out cos dont no if its deff problem with the gearbox or am i thinking it may be a sensor?? great for any advice anyone can give me...

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