i had the same issue with my jb4.. EXACTLY. I would describe it as a shudder during shifting. I first thought it was tranmission but then found out most of the time it is old spark plugs. not being able to deliver the precision needed during heavy shifts, believe it or not. read many accounts of this. since have gone back to stock but just FYI i am pretty sure i know the issue.
2008 135i | AW | Sport Package | Premium Package | 19" Matte Black CSLs 225/35r19 | Comfort Access
Adaptive Xenons | Nappa Leather | Heated Seats | Power Folding Mirrors | Rain Sensor | Clear Bra | Mint