What Causes Fps Variations?

I've been getting microstutters lately. I know what is causing them (I think) but I don't know why it happens. I run FPS unlimited and I get huge FPS variations, like sometimes up to 40% off the average, as indicated by my FPS LOW AVG Hi meter. Why do the FPS vary so much? I believe the huge variation in FPS causes microstutters. IS this correct? At first I thought the stutters were when I changed views a lot, or looked all around my aircraft. But I found they even happen when I'm looking out the left window for 5 min straight. I typical scenario for me is the RealAir Scout at 100kias 3000ft. I look out the left window for 3-5 min. The microstutters will continue throughout the entire period. They usually subside after 5-10min IF I'm flying in the same area.