Everything you described points to a failed charging system as the main suspect. The most common cause is worn brushes in the generator, and those are very often intermittent. Any testing must be done while the problem is occurring, otherwise everything will test okay.
You also have to look at what tests were performed. A lot of auto parts stores, which are not repair shops, and often don't employ mechanics, only test for output voltage. If that test passes, it only means there is SOME output from the generator. If one of the six diodes fails, you will get a maximum of exactly one third of the generator's rated current, and that's not enough to run the entire electrical system under all conditions. You'll only get 30 amps from the common 90 amp generator. When you turn on more loads, especially big ones like the heater fan, the battery has to make up the difference until it runs down. Sounds like that is what was happening on your car.
The rest of the test requires a professional load tester that will measure the maximum current the generator can develop, and "ripple" voltage. If one of the diodes has failed, that maximum current is going to be in the area of 25 - 35 amps, and ripple voltage will be very high.
Also, with one bad diode, since there is still some current output, the warning light on the dash may not turn on. If you have a voltage gauge, you'll see that drop when you slow the engine and when you turn on more circuits.
Saturday, December 21st, 2013 AT 1:14 PM