I've heard of a few cases of frame stuttering with X-Plane 11, and have read that the LR development team are looking into the causes (which could be down to a number of issues - for example, plugin conflicts or OpenGL problems).
The X-Plane.org forum site have a technical help section which could help to troubleshoot your case. Doing a basic topic search via the forums does give a number of responses of people with the same problem, and in doing so I found a great post that might be of benefit:http://forums.x-plane.org/index.php?/fo ... nt-1205660
In the above post, "JustSid", who works for Laminar Research, has asked users with this issue to contact him so he can look into the code a bit deeper and get to the bottom of the problem. If anyone could help, then he's probably the best person you'll find! X-Plane 11 is constantly being updated, so your assistance could help others who are affected by this.
Let us know how you get on...