Patrick, I had similar in my 757 last night, after positive climb, but before 1000 AGL flap retraction scheduled, the plane kept pitching up and up and up and finally stalled before I realized just what was happening.  I had very poor weather, and thought maybe this was the cause.  Reset back to the active, reloaded the plane, reset my FMS and all checklists and proceeded again without any trouble.  So I don't know, but worth comment that I had a similar thing there.

Haven't had the LNAV/VNAV issue the original poster mentions, but I have to wonder, what was the condition of FMS in the LEGS screen when it happened?  Unresolved Route Discontinuity will cause that sort of behavior, I think.