New Gen 737 Lnav Vnav

[fryingeggs]

Since acquiring a b737-800, and flying pass the 180degree meridian, on a few occasions, the LNAV and VNAV have both dropped out, powers have come back to idle, unwarranted turns initiated, and a variety of other unexplained actions.
i would appreciated it if anyone could give me any insight into what is happening when crossing the 180, as this never happened on the prior generation of 737's(300/400/500)
thanx.