Yeah the Ultra setting is bad for DCS and causes very irregular FPS, a good explanation of this feature is written here
https://www.anandtech.com/show/14765/nvidia-releases-geforce-436-02-driver-integer-scaling-and-more
"Ultra, meanwhile, is more -or-less new, and goes one step further by reducing the queue size to 0, meaning frames are submitted to the GPU on a just-in-time basis and no pre-rendered frames are held in reserve."