My 1990 Wrangler with a straight six and 5 speed has developed a mysterious problem. When it starts from cold, voltage on voltmeter shows about 14 volts. After driving for about 20 minutes, indicated voltage drops to about 6-8 volts. However, measuring voltage from alternator (output bolt to negative battery terminal) still shows about 13.5 volts. Alternator was bench tested at autoparts store, they say it's OK. Belts, connections seem tight. I have cleaned battery terminals, but this is the second battery it has eaten. Some guy said maybe my fusible link is going bad. Unfortunately, I don't know where that thing is, or what it looks like. Anybody out there have any ideas? Thanks.