I had some problems (many problems) with my 320i e30 1988's ac.
one of them is that when I turn the ac on the rpm drops by about 200 , and when i drive the car is ok, but when i stop for a traffic light the car dies bcz rpm is getting too low.
first I have adjusted the tps screw, the car was ok when turning on the ac, but when i turn off the car and switch it back on with the ac switched of the rpm got to about 1500 - 2000.
so i changed the fuel pressure regulator and it's hoses, and this problem was fixed.
p.s i had to re adjust tpsafter changing the fuel pressure regulator.
Hope this wld help.