One other item to consider is your altitude. You need to de-rate the generator capacity by 3% for every 1,000 feet you are above sea level. At 5,000 feet, for example, your 1600 watt generator becomes a 1400 watt generator.
Also, try running the generator with the "eco mode" off (or whatever it's called.) That should force the generator to run at higher RPMs before it is loaded and reduce the impact of the microwave's start up draw.
One last thing, make sure the oil level in the generator is topped off.
I've personally learned all the above over the years by direct experience. ;-)