I'm not sure what you mean when you says goes curvy. If you are referring to the curved motion path then that is normal with the default preferences setting for motion paths to be bezier. You can select the pen tool (letter g) and then hover over the keyframe with the bezier handles and you'll get the convert point tool which looks like a sideways V, then click to remove the curve. You can also change the preferences on the general tab so that spatial interpretation is set to linear. Changing the preferences will only effect new keyframes.
Sorry about my english, I didn't explain it clearly. But it was right. I already changed the curve path. Thanks a lot!!!