Ok, if I understand what you are saying you have two seperate issues going on here. The first is that the car is twitchy under hard braking, the next is that it is now twitchy all the time with the bigger wheels. Lets look at the second issue first: Your problem has gotten worse with the addition of larger wheels. this tells me that either your car is tramlining simply becuase some BMW's do that with wheels that are larger than stock, or the addtional size and i imagine width is generating more stress on excessivly worn control arm bushings.
So the first thing to do is to make sure thast all your bushings are in very good condition. after you have seen to that, it is time to look at the brake system. first thing to do is a complete flush and bleed. By the way, if you gravity bleed or vacuum bleed you will need to do this procedure twice or three times over the course of a couple of weeks, since either of these two methods do not bleed the fluid from the ABS accumulator. So flush and bleed brakes, go out and drive the car making sure you activate ABS several times, then do it again. if it has been more than three years since a complete flush, do the whole procedure one more time.
I also recoommend using BMW rotors from a dealer? yeas i know it is hard to beleive but hear me out,,BMW, when driven hard, warp rotors .. that is a simple fact. BMW will warranty your new rotor for one year. buy them using a platinum charge card and that warranty shuold get doubled to two years.. For rotors...... now that is a deal.
One more thing E36's eat outer tie rod ends also... I am assuming you checked this early on right? If not , that is the FIRST place to look, as this will introduce a whole bunch of slop in the steering . let me know what you find