Microwave oven and electric Range in operation

Are they on "separate circuits"? Or is it remotely possible somebody has wired two outlets using some sort of common cabling back to the circuit breaker panel? For example; if a 115 volt outlet for the microwave has been tapped off one side of a 230 volt circuit for the electric range? Not to suggest the OP is wrong but just making a suggestion as to cause. If it Is due to voltage drop, sounds dangerous? Possibly wiring gauge is insufficient somewhere? Unlikely but do you have a bad/loose neutral connection in the circuit breaker panel; or something crazy such as someone using a ground wire instead of a neutral?

Voltage drop, but now you have to figure out why. It is possible you are not getting full voltage to begin with. It is possible you have other loads on at the same time, like central AC, clothes dryer, power tools. You may have a bad connection. At a minimum, you need a meter to accurately check the voltage and go from there.