English isn't your first language is it?
If I'm reading this correctly, you're seeing an instant spike in voltage to 15.1 volts when you put a load on the system (windows,etc), but then it instantly drops to 14.5 volts?
It really doesn't sound like anything is wrong. The alternator has a voltage regulator built onto it. When you increase the load on the alternator, it is forced to output more amperage. But the regulator is probably just trying to compensate for this instantaneous increase in load. It's a balancing act. A tiny split-second spike in voltage like that doesn't seem odd to me. The battery should help smooth out spikes/ripples in voltage. I guess it's a possibility your battery is a little old and maybe not doing this very well, but I really don't see a problem.
I'm not sure what you're trying to power with the 220V power inverter. Usually they're not good for anything more than charging a battery to a personal electronic device (laptop battery,etc). And they're usually useless for anything more than about 10amps. I'd try a different inverter. See if you still have problems.