I used to have a 1990 2.6 as well and
suffered from rough idling.I did a major tune up and still the problem exist.I gave up and took it to a MB mech.He found that my idle valve is bad.
I taught it was the injectors too,but he said,
if its the injectors,the car will hesitate on
accelaration.BTW.Plug wires are brand new.