1994 Ford Explorer Haevy Battery discharge after starting

The problem is intermittent. The car starts easily, then I notice the ammeter is showing full scale discharge, with lights on or off. Most often, after driving for about 15 minutes, the ammeter will suddenly return to normal. Just yesterday, I had driven about 10 miles with the car starting normally and showing no problem until I parked the car to shop for food. After shopping for perhps an hour, I returned to the car. The engine then started easily, but I noticed that again the ammeter was showing full discharge. This time, the ammeter continued to show discharge until I had driven home and turned off the engine. This morning, I started the car with the ammeter again showing full discharge. In about a minute, the ammeter suddenly returned to normal. I drove about 15 miles with the ammeter showing charging of the battery. I returned home with the ammeter still showing normal indication. What is happening? Starting relay or solenoid sticking? How to diagnose?