The voltmeter will drop to around 8 volts while the engine is running. The alternator light does not light and the bulb and resistor are fine. I replaced the battery in June when it gave a bad load test. In August the alternator wouldn't keep a proper charge on the battery, so I replaced it then. Everything worked fine until 3 weeks ago when the voltmeter indicated low suddenly when the car was idling. I had to drive it 20 miles home and the battery was completely spent just before getting there. I charged the battery fully and called a place that does some electrical repairs and to save the tow charge decided to drive the 6 miles there. 2/3 of the way there, the voltmeter showed a full charge on the battery and the car sat there for 3 days just fine. Picked up the car and drove it to work and the voltmeter dropped out a couple of times after I was too far along to turn around. It made it home that evening. I can't trust the car as it is. I have replaced the alternator again the weekend it first acted up and it works. I can't find an open in the wiring and I can't get it to fail on demand so as to see what is doing this. I take it out on errands with a voltmeter in the car but it won't fail except where I can't stop to measure. It failed in the driveway and i started checking around the alt and batt and everything read normal. It had started working again. Any help would be greatly appreciated.