If the output voltage is no much over 100 with no load, then it's not a reving-up problem. I suppose it could be something simple like a bad internal wiring connection. My guess is that probability-wise, it's something not worth the cost of fixing like the alternator or the inverter. I view it as a roll-the-dice and see how you come out proposition. If you don't have electrical troubleshooting experience and an oscilloscope I think the deck is stacked against you. For some it would be worth the cost just for the learning experience.