low voltage on power inverter

My problem im having is my 3000 watt power inverter is showing it is only getting 8.4 volts, and will not operate under 11 volts. I tested to voltage on the cables running to the inverter and they are reading 12.44 volts. The cables are 0 gauge about 7 feet long. What could be the issue, and is there a way to fix it by my self. I have some electronic knowlegde. need a fix by October 2012.

power inverter model Whistler PI-3000W

three Marine 12v batteries in parallel 375 amp hours combined

Check for loose or dirty connections at the inverter.