Just a few thoughts on your brakes. Many of you 'newer' owners of BMW's are unaware of the problems BMW had with warping rotors in the early to mid '90's. IT effected E32's E34's & E36's and it took a few years [and probably $100,000's of BMW warranty money] to trace the problem to brake pad composition and finally correct the problem. Once in a while I've come across an E23 like yours with this problem but it's usually the fronts and I've found it to be either an 'aftermarket' wheel problem or a bent wheel hub as the pad problem didn't effect this chassis much.
You mention the rears having been 'turned' twice now!! Working on these cars for years has taught that if the rotors are worn enough to 'require' machining, REPLACE them or you will be anyway as the thinner they get, the quicker they'll warp due to the lost mass to dissipate heat quickly enough.
You didn't mention whether the steering wheel is shaking when you brake, more info might be helpful.