You have to explain a bit on rough idle. If by rough that it shakes the car, and smooths out with higher revs, that could be your balancer. If the tach hunts for idle, that'* IAC. If it doesn't sound smooth at the exhaust, that'* uneven compression/ turbulent airflow [carbon caused]. You've got the basics checked out... but you do have low mileage [low chance of low compression, along with carbon buildup]... could be injector caused. Leaky injector by chance? Good O2?
Again, vacuum lines can be the cause as well. Vacuum gauge it, and see. 18inlbs right on the dot for me. So it should be exactly the same for you...
'89 SSE - GX3 -190k, 155k on body - Back for another ride.