Its possible that the battery was nearly dead, ie its holding sweet FA charge these days. Put in a stereo and she really struggles when ur engine aint doing enough revs for the alternator to bt fully supporting everything. Hence the low idle. If this is the case, adjusting idle will only mask the symptom. That is; pump in heaps of extra air & fuel to compensate for half a spark...??? not a good solution for my mind...
Go to a batteryworld or something and see if they think it could be the cause. In saying that, dont walk in asking to hand over your money, get them to justify replacing it too!