Weird electrical problems

[fiat128 at mindspring.com (Wiley Robinson)]

Hi all!  I have an 81 S that I bought from it's original owner.  The car
has had some recent issues with keeping the battery charged.  So far I've
replaced the battery, then the voltage regulator, then the entire
alternator and lastly the wiring harness from the alternator to the
battery.  The car seems a bit better but I can't use it a night without
carrying around several extra batteries.

I getting really frustrated at this and would greatly appreciate any help.
I really like the car but must fix this problem.  I can tell the car is not
charging correctly because the gas guage drops down as does both the water
and oil temp (really scary not to have these two working properly when you
live in a place as hot as El Paso!).  Also this week, the seatbelt light
has started flickering on and off and the gas mileage is getting worse.
Seems really weird to me.