I keep close tabs on my electrical power with a digital display that is next to my kitchen sink. If it goes below 105 volts, I will shut off my A/C unit.
If it is dropping suddenly, like when the microwave is on, then that indicates a loose screw that needs to be found. If not by you, than someone qualified to check it out.
You can buy a 'Kill-A-Watt' for about $20 now, and it not only has a digital voltage display, but you can check the wattage of anything you can plug into it. Yes even the built in microwave if you can find the power cord, and plug it into this device. Check Amazon.com and search Kill-A-Watt - there are several vendors selling the same thing.
Good luck,
Fred.
