This is fairly simple if I understood it correctly, but involves a bit of IPO work. Have the object follow the fath using the follow path constraint. Once this is done, go to the constraints menu (under the objects header, F7). There should be a box with the information for the path constaint. at the bottom, you’ll see a slider marked influence. We need to animate this with an IPO. by default, there is no curve for influence. To create one, click the button marked edit, next the the slider. Now, open up the IPO window and choose IPO type Constraint. the inf IPO should now be editable. Simply have the curve be 1.00 when you want the object to follow the path. Have it go to 0 when it stops following the path. (note, when the influence drops to 0, the object will go back to its original location, or if you added keyframes, wherever the keyframes tell it to be at that time [the path constraint overrides keyframe positions at strength 1.00] so on the same frame the inf goes to 0 add a keyframe there so the motion is continuous.
Using and animating the influence is a pretty easy way of setting up multiple parent relationships. You can go from path to keyframe or path to path animation or have some more complex things with influences between 1 and 0.
hope that helped.