Hello, i'm driving a 320i e36 146.000 km's and sometimes 1st gear pops out. I have heard different things, like chance the tranny fluid, adjusting something or even replace the whole tranny

Does anyone have an advice for me? The rest of the gears is fine even the 1st works fine most of the time, until it pops out.
I have to say great site, good advice and great cars!