89 toyota pickup electrical problems

Several months back mt headlights would get very brigth then go very dim, the air would go from blowing really hard to barely at all i was thinking my altenator was going bad. All of the sudden the problems stoped for about a month. Then while driving down the road my stero shut off and lost power. About a mile down the road my truck shut off completly yet my headlights were still on but very dim. i pulled to the side of the road and checked under the hood. Was unable to notice any problems. Got back in and it wouldnt even let me turn they key no matter how much i wiggled teh steering wheel. I left and came back to my truck abiout an hour later and it was if nothing had even happend started up fine. Drvoe it to a friends house near by and left it over night. I came back the next day and drove it home. About a mile from my house the stero lost power again. i pulled over and turned off my head light and wind shield wipers and air. It was still running. If i even turned on the windshield wipers it would start to die. I was able to get it home. I have had both the altenator and the battery tested and was told they were both like brand new there was no reason to replace them. I was told this by two different places. I have no idea what is going on please help. This is a 1989 toyota pickup 4wd 4cyl. with a 2.4L 22re.