FMC Problem

So, I'm flying in South America, enter the flight plan in the FMC, and take off. Enable LNAV and VNAV, and presume I have a 5 hour flight to do homework and then bring her in for a landing. Not so. I hit a thunderstorm, and then FL CHG activates, and she starts descending. FMC and Altitude both set for FL 320, and I'm at 22000 coming down constantly. I hit VNAV and it switches right back to FL CHG. I can't find anything about what FL CHG does or is for in the documentation, and I'm getting quite frustrated that A) Altitude Hold won't even stay on, and B) I am still 500 miles away and the plane is bent on descending into the ocean. How do I turn this off without manually flying her for an hour and a half?

Oh, yeah, the FL CHG took us on a 6000 ft/m "dive of death"and the stall warning won't go off even though I'm 50 kn over the yellow warning line. Is this a bug?