Is the cap on the thermostat neck spring-loaded, or just a cap? I had a similar problem with a replacement cap on the thermo neck; the spring-loaded plunger would open slightly when the system reached operating pressure & bleed off some coolant, which would then leak out from under the cap and drip or evaporate. The overflow on the radiator would work as intended, but since you were also bleeding out some coolant and could be sucking air back in, the coolant level in the engine would fall. I eventually found a spring of similar size to the one in the cap on the thermo neck and wound it in there to increase the pressure rating of that cap, so the radiator one would open first.
Check if the hose from the rad overflow to the bottle has any small cracks or holes that could be letting air in, as well as the hose/piece of the overflow bottle cap that goes to the bottom of the tank to suck coolant back in. leaks in these pieces would allow coolant into the overflow but not back.
Are the voltage fluctuations random, or do they occur under specific circumstances?
