More definition of rough idle needed:
1) Steady roughness, as though idle were too low?
2) Idle irregularly rough, then smooth,?
3) Idle speed irregularly fluctuates up and down?

I'm a bit puzzled why you had no success getting air out of your system
with the bleed screw. I drain and fill my '91's cooling system every year,
and have done so for 12 years. Never a problem of any kind.

I wonder if you have a bad head gasket. When you drive it, do you see
bubbles in the resrvoir? Does the reservoir level climb, then not go back
down again when the car cools off?

Does the temperature gauge ever climb higher than 2/3 of the way? Does it
never get to 2/3?

Do you have an aftermarket thermostat?