I have a 94 525i and I have a similar problem. I took the car to my local indy and he ran some bg44k to clean up the injectors and combustion chamber. It was fed directly into the fuel rail.

It helped some, but there is still some ping. Not sure if there is a real solution to completely get rid of the problem. If you want something more DIY, you could try Seafoam. Just search for it.