advise its not the gearbox, whoever done the inspection has not done the rest correctly, take it bac to the place you had it done will need to have a diag done.
i also have a E30 Convertible 320i for sale i find that buyers still prefer 323 or 3251 i find the power is still not as fast from stand still as my 325.
Bit pricey all ways find a good tech then you will find work on your moto gets cheaper as most good techs resp good customers who keep thier motos up to spec.