electrical issues are hard to diagnose, let alone to tel you where to start over the internet.

based on your description, I agree with you about the bad ground. At less than 11 volts, the gauges will act funny, the car probably won't start, etc. This could be a bad battery, corroded ground wire, poor connection, etc. So try adjusting the ground terminal. Have the battery checked, and check the main harness grounds/wires and connections for corrosion.