2003 Toyota Camry with charging difficulties

[ToyotaCamry22]

I went to my local shop (which I highly respect and recommend) with a difficulty of a 2003 Camry having difficulties maintaining an adequate charge from the alternator.  At certain times, the alternator starts slowly dropping charge down to about 9 volts.  Not all the time.  And it usually seems to happen after the Camry has been running for a while.

Once the situation begins after starting the car, it will continue to lower the voltage to about 9 volts and then keep it there while at idle such as at a traffic light, car pickup of children after school, in fast food lines, etc.  

When you go forward again with the foot on the gas pedal, the voltage will also increase back to the normal range until you come to another traffic light, etc.

If you stop and let it idle for a while when the voltage is low and then decide to turn off the car, then the car may start with some difficulty or not at all.  But if you continue to drive it and then turn it off, the car will easily restart again.

I have an inexpensive charge monitor in the 12-volt outlet which helps me track what it's doing between shop visits. 

The shop I go to has tried a number of ideas but nothing seems to fix the random moving to a lower charge voltage even after three alternators have been installed.  The first one was replaced, and then they tried a different brand thinking that possibly the rebuild plant which had rebuilt the first two alternators probably did so on the same day and may have had a production issue.  The third one is from a second vendor.

It does have a recent battery from August.  From the times that it was in the shop, they would have surely checked its ability to take and hold a charge many times over and the various battery and alternator connections would have been checked.

The shop has offered to fully refund the cost of my original repair ticket and which included diagnosis and alternator.

The question is:  what would cause the alternator at random times start to provide a low charge, and then keep that low charge during idle and higher charge while driving?  Whenever the situation begins, it continues until the car is turned off. 

Thanks in advance for your insights.

[Toyota Owners Club]

This can occur when the engine speed drops too low, the voltage regulator drops down to a low charge rate but increases with revs. 
So thoughts are:  voltage regulator faulty, earth leads faulty or corroded and also alternator pulley size is incorrect (got caught out by that one many years ago)

Also check the idle speed is correct and if possible rectify an issues that are causing the engine to run at a low speed