Electrical Issue with 1997 Jimmy?

4.3 L 4WD w/ 105,000 Miles. Issue I have is sometimes when I go to start it, the engine doesn't crank over, I lose all dash/dome lighting, and the radio presets all get wiped out. I turn the key to the off position, and then I try to start the truck again. Sometimes the above process takes place again, and sometimes the truck turns over and starts. I have a car starter that has never given me problems, and I also have an immobilizer system installed by a certified shop. I have tested the battery at 3 different shops on 6 testers, and have taken in the alternator for testing. The battery, and the alternator are good. It should be mentioned that when the above process takes place, a click is heard. When power returns to the dome/dash lights, a click is heard again. The guage indicates a good charge when it is running (over 14 volts), and there is no issue once the truck is running. Occasionally I get a flickering of the dash lights while driving, and the headlights while driving at night. I have also cleaned all the battery connections, and the terminals. I am at the end of my rope, please help!