NOT the usual suspects

I was driving my 2001 Ford Expedition when suddenly the needle on my voltage indication started going down as I drove.  I turned off everything I possibly could and made it home just as the needle hit the bottom of the red.  

I thought it was the alternator but my mechanic gave me good news and told me it was just the battery.  The next day, it started to do the same thing.  When I reached the store, the needle was in the red.  When I came out and tried to start it, it sounded like it had trouble but did start-- the indicator was absolutely normal again.  This time, the mechanic replaced the alternator.

The next day. I started the car and moved it so my wife could get out of the driveway.  Minutes later, the car wouldn't start at all again.  Nothing electrical worked-- exactly as it had been each of the two previous times.  The mechanic jumped it and tested all systems-- all worked fine for him. The next day, I went through the same thing again!!!

We are both in school and have a newborn daughter--- I can't afford to keep chasing the wrong solution and I can't afford to have a car I can't depend on!  Can anyone point me in the right direction????

Thanks!!