Some other ideas--..You might want to pull a plug wire with engine idling one at a time to see which cylinder or cylinders is weak/dead-The RPM wont drop off when you pull off the wire to the problem cylinder or cylinders. Look for a bad spark plug/vacumn hose off,stuck valve ect.. You should probably look at the plugs now and if they are black replace them don't clean them-they should be tan in color. Coil also could be carbon tracked- high tension tower to either primary stud-shows up usually under load. Compression test might also be called for . More than likely its plugs or points/condensor issue-simple things 1st!