charging

ok...i had the "fuel problem" yesterday, and have some electrical problems today.
I connected a spare battery with cables to the power junction uner the hood to help start the car. got it warmed up and idling at 800 rpm. had 13 volts. idled it up to 2K and had 14 volts. brought back down to 800, and disconnected battery. tach started flaking, and voltage was at 12.5V. connected electric fan and voltage drops, car dies. restart. idle up to 1000, reconnect fan. everything's good. disconnect battery, voltage drops .5V. reconnect battery, drop to 800, disconnect battery, car dies. disconnect fan, reconnect battery, restart, disconnect battery, car dies.
(when car dies, i mean voltage fades away until car dies)

SO...it seems that even without the fan connected, sometimes voltage drops, and sometimes it doesn't when the battery is disconnected. and only at idle...seems to be fine at higher rpms.....in the garage.

can the alternator be working sometimes and not sometimes? i have always had fords, and anytime the alternator went, there was nothing...it wasn't sporadic.