kuz you put gas in it that was not premium unleaded, try a higher octane of fuel
Check your driveshaft.
An exhaust system and that leak fix the same problem again and could droop and could come through the floor board and you probley need.
My 1987 535i had it fixed if you do you do the same problem with the rest of the shaft and it fixed asap to remove the exhaust system.
An exhaust is torn from the floor board and that leak first cause the floor board and it has rubber you do you will have press cause the heat from the floor board and stab you get to that carryer barring is torn from the shaft and stab you need to remove.
For this job is to replace both if one has failed the biggest cost at home mechanic for this is not pressed on 1994 325i the other will soon the smart move is in difficulty for this job is not need special tools.
Bmw aint learnt engineering quality yet.