Lnav is having some issue after making turns that it over corrects horribly then thinks its on course when its not even close have a look.

Check here:

http://forum.avsim.net/topic/339174-pmdg-and-the-s-turns/

 

http://support.hifitechinc.com/Knowledgebase/Article/View/11/4/how-to-prevent-s-turn-issues-with-certain-aircraft-autopilots