1979 Voltage Spike

[thumpernickle]

I am not much of an electrician and I am puzzled by a problem I am having with my 1979 Chevy pickup. My truck starts up fine in the morning. I drive it about 4-5 minutes with my factory installed voltmeter reading about 13-14 volts. After the 4-5 minutes, my voltmeter jumps up 17-18 volts and usually stays there for the rest of the drive. Sometimes while the engine is running it will drop back down to 13 range and then jump back up, other times it stays pegged at 18. The headlights seem to decrease in brightness when the voltmeter reads 17-18 volts. When I have driven it like this for more than 10 minutes and then immediately try to start it, it is dead electrically except for the lights. If I let it sit for 15 minutes I have electricity again and it will start.

My battery shows signs of overcharging. It is obviously leaking fluid from the top (it is a top post battery). The positive battery cable clamp has turned black.

I had my alternator and battery tested at Kragen and the they tested "normal."

Any suggestions of where to start looking for problems?