MY 757's head is vertically unstable after descend. Everytime, when it reach the altitude you set on MCP, the Autopilot didn't stay at LNAV and VNAV pitch. Autopilot change to ALT and the head keep up an down, never stop. the worse result is I can not LOck and App, sometime, after Lock, the plane heading a line parallel to the runway, and then steer lift and right...
How to resolve ?.....

Without more info on how high you are, what your target altitude is, are you doing an ILS approach? Is the ILS System Set up, is the Runway Course and frequency set in the Radio stack. Are you intercepting the Runway below the Glideslope?

Try this:
http://www.utem.com/pdf%20Downloads/article%206%20AIRBAG.zip
Its a free supliment.

This time I am very sure that the unstable situation started by the pressing to reset altimeter during descending or between descending stages. Does any Vpilot know what mistake I made or met the same bug of game? if it is due to my mistake, how should I do?
thanks

This is realy weird.
The only other thing that I can think of is that the plane might be overweight. Looking at the manual, verify your takeoff weight and your landing weight. Check your trim settings. I have seen other FS (Default) Aircraft react negatively when you are landing and using speedbreak. they will oscilate badly. Check the Vref Approach page and see what your flap settings should be.