I would have them check out the IAC valve (Idle Air Control Valve). Carbon deposits can build up on the inside of the valve and effect normal operation. Sometimes the valve can just be cleaned, and other times, it may need to be totally replaced. A service mechanic should be able to determine if the valve is functioning properly and move from there. This is just one suggestion, but lets see what others suggest.
I agree with Rob. If the IAC sensor is not functioning properly, the car will still run great under throttle, but it just won't idle properly. If you do need a new Idle Air Control sensor, they're pretty inexpensive. Good Luck.....
YOur theory on the Optispark may be correct.
Find a shop, with either a Tech 2 scan tool, or a digital storage oscilloscope.
Have them monitor the low resolution signal.
If this signal drops out, it defaults, thereby increasing the idle.
IAC can also be controlled, and monitored, however, you may find that the IAC is being commanded higher by the PCM, as a result of the missing signal.