Make sure older version still works. Anything else changed since ? the driver, the card ?
If it is really the code, then diff is your friend.
Such low poly model should have no performance difference between strips and triangle lists.
After reading the opengl specification PDF, I think I have found the solution.
As you have said, openGL determine the front or back by screen coordinate. As stated in the openGL spec., collinear triangle is treated as BACK faced. I think it is the reason those LINE appear as I draw BACK face triangle. I just wonder why it works before.
I will try another method to draw the outline:
draw the model again with LINE mode,
change the orientation of front-facing polygons to opposite (CCW to CW or vice versa). Then the BACK faced polygon will change as FRONT, but the collinear triangle will stay as BACK (since its area is 0), cull back-faced and draw.
I need to verify if it is correct by next day.
Thanks everybody! 
Finally, the problem has been solved by the method I memention above.
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