If you can get it to act up again, don't wiggle anything. Try to keep it in the bad state so there's a defect to find. Use a test light with the ground clip on the positive battery post, then probe the negative side. Unfortunately GM made diagnosis difficult with their side post batteries. You may find voltage on the negative bolt head but that doesn't connect directly to the battery's terminal. Move the test light's probe to the engine block. If it lights up there, move it to a paint-free point on the body. If there's no voltage there, check the smaller negative wire that bolts to the body. In this case a test light will be much more accurate than a digital voltmeter.