Ford Repair: Aerostar Low Voltage Reading, alternator failure, ford aerostar

Question
I have a 1993 Ford Aerostar that recently began showing a lower then normal voltage on the dash gage. The charge light is not on but I initially assumed an alternator failure and installed a replacement.  Unforturnately I have the same voltage reading now, still no charge light.  My battery strength seems normal, no dimming of lights under load.  When I initially start the engine voltage goes up to around 12 -13 volts, this is what the normal reading was before, then slowly decreases to around 8 - 9 volts.  Engine idle speed also noticeably changes when the voltage drops.  The low voltage reading is still within normal limits on the gauge but is definitely sevarl volts below what I've come to assume is normal.

Answer
disregard guage have checked with a voltmeter to verify low voltage
you've already thrown an alternator at it to no avail-have complete
charging system checked including battery a weak or bad cell or dirty connection can cause this you may want a shop to check this