Do you mean the flange/yoke nut is constantly coming loose, or is the nut tight, but still slop existing in the driveline?
If the nut is constantly coming loose, clean the threads on the shaft and nut, add some threadlocker. vibrations in the driveline can cause nuts to come loose unexpectidly.
If the nut is not coming loose, and the shaft is actually able to wander in and out, you may need to install a new front output bearing.
The thrust loads from the helical cut gears in a 205 will eventually wear the front output bearing causing slop. Some slop is acceptable, but if it is excessive I would recommend replacing the bearing.
-Kyle.