Hmm. A bad ground connection usually causes the needle in a gauge cluster to fluctuate unnecessarily. It is possible your ground connection got a bit too corroded and is causing your problems. If you know where the ground wire was previously tightened, try cleaning up the oxidation around that connection and seeing if your problem goes away. If it does, hit that spot with spray paint to prevent further rusting. It is possible the location on your wiring harness that connects from the (-) on the battery to the frame is corroded or a part of the wiring harness insulation has become exposed.
Remember, a car battery can kill you. When working on a car wiring harness, make sure to remove the wires from the battery terminals and isolate the battery terminals with electrical tape. Sometimes it's a good idea to replace the battery terminals if they become too corroded / oxidized or deformed.