Hello,
I have the same problem with the 767 (and 777).  I just bought the 767 cuz it was on sale and because my CS777 started this same loss of hdg and LNAV problem, except altitudes are different as you all describe above, with the added irritant of a BSOD everytime the 767 (not on 777) is used  (BSOD even happens when 767 isn't used). If I uninstall the 767 the BSODs go away - in all cases.  By the way, I haven't had a BSOD in years.

During install all anti virus and firewall are disabled.  FSX is on its own SSD.  UAC disabled.  I've been doing FS a long, long time and this problem has me stumped... 

The 777 AP problem started back in September 2015 -- suddenly, for no apparent reason. (I've had that plane from the beginning version and the AP has always worked through all versions -- until 09/2015).

I hope we get lucky!!!
rgds to all,
Walter