Charging issue.... I think
Charge the battery if necessary, start and warm the car until warm, then bump up the idle to simulate "medium cruise" IE 40-50mph or so
Read the battery voltage. Should be 13.8--14.2, in no case below 13.5 or above 15
Start turning on accessories. Lights, heater, radio, jumper the fan if necessary to get it on, IE turn on every load in the car you can
Now measure the battery voltage, AND measure the voltage at the alternator output stud
Post the readings here
Are you running factory wiring, IE you still use the factory ammeter?
If the voltage at the battery goes low but remains fairly normal at the alternator, you have a voltage drop problem in the charging line, and your top suspects are the bulkhead connector and the ammeter
Read this MAD article on the pitfalls of the bulkhead connector, and in some cases, the ammeter:
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
If the voltage drops at BOTH the battery and alternator, something is wrong with the alternator (one or more bad diodes) or the alternator is not large enough
OR THE BELT IS SLIPPING