First of all I will tell you that contrary to postings on the web that stepper motors made for other systems are the same as the one on the RRC are the same is incorrect. They are not the same, and you can easily confirm this with a multimeter and by direct substitutions while observing idle quality.
You may have a problem with the road speed sensor. If the RSS is not sending a pulse(vehicle is moving), then the idle control system is turned on(vehicle is not moving). I think it's unlikely that the RSS is the fault, but that is what turns the idle system on and off, so check it thoroughly.
The code 48 may also mean that your engine is faulty and now the stepper cannot do it's job correctly. That could be cause by using the wrong spark plugs, wrong cap and rotor, etc. That may also be caused by the air gap in the distributor being incorrect. That may also be caused by a dozen small faults in your engine which are conspiring to produce a poor idle. Idling is when most minor faults manifest themselves.
Randall
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