It sounds like you are working with a 12-13ms refresh rate. If so, the your displays will either be on for say, 12, 24, or 36ms. If there is a lag in your display hardware you can compensate by subtracting off ms from the requested values. In your case though you are getting 24 sometimes and then 36's. I can't say for sure what is going on, but one thing I would try is changing the requested value to 15-17ms. High enough so that DirectRT will not try for 12 and low enough that a few ms of display hardware lag will still make 24ms possible. It's unusual to have to do this, but you can, at least, count on the log file data to give you the actual display times so you can play around with stuff like this.

See also: http://www.empirisoft.com/Support/showthread.php?t=55