erratic idle 87' 528

My son's 528s idle is very erratic. i have checked the idle stabilizer valve according to the Bentley manual and have found it closes with power andhas 10ohms of resistence. The idle remains erratic even with the wires to the ISV disconnected. I cannot find any vacuum leaks. What is next?

I have an '88 535is which had the same problem, The thing that fixed the problem for me was replacing the fuel filter at the left rear of the vehicle right in front of the gas tank,if that isn't it take a look at the fuel injectors maybe run some injector cleaner through the car...i did...finally if the problem still isn't fixed check the distributer, plugs and wires (make sure they are gapped correctly and tight and all that)

not sure if your 87 supports engine codes but thats where I would start.