757 uncommanded descent
In the climb for FL340. VNAV engaged. asked to pass FL310 10 miles before....... selected full climb, speed intervene.
Made the level, no problem.
closed the speed window, selected climb 1.
Next thing I know aircraft was in 1500fpm descent. Took out automatics, eventually fd's caught up. re-engaged c cmd, flight continued normally.
Anyone seen this before?
If so, know why?