[Oldsguy]

I think the voltage drop is a normal indication of the lower idle which means the alternator would develope a lower voltage but not abnormal. I work at a Motorola radio repair shop and we deal with a lot of vehicles. We consider normal vehicle voltage to be 13.8vdc, knowing some are higher and some lower. Don't worry about the voltage. I think C.J. is onto to something. In particular, check that vacumm from the manifold vacuum switch, it probably is supposed to change the amount of vacuum based upon engine temperature and it may produce a vacuum leak when the engine warms up. Check it when the engine is warm. Good luck.