I would be that the spatial interpolation is set to bezier. Here's what I would do. Take a look at a top view with the camera selected so that you can see the motion path. Now press the G key to bring up the Pen tool or click on the Pen tool and hold until you reveal the convert vertex tool. It looks like a sideways V.
Now click on the middle keyframe in your comp window. You can't convert the keyframe in the timeline, it must be done in the comp window. This will convert the keyframe to linear. That should do it.
I'd know if this was the problem for sure if I could see the graph editor with position and speed values displayed. I'll bet it looks something like this:
You could also fix your problem by selecting all position keyframes and changing the Keyframe Interpolation to Linear. You'll find it under Animation>Keyframe Interpolation. This will convert all selected keyframes to linear which will probably be OK for this problem but it may foul up other more complex moves.
hey thanks Rick.. i follow your instruction which you said in the paragraph one. view with the top view and use the pen tool and select the point of position.. and my problem solved.. thanks..