That is a tough one. My best guess is that there is a bushing that holds a significant load, such as the trailing arm bushing.
When the car is cold the bushing is brittle and possibly moves more.
This kind of goes against physics becasue as it gets warmer, it should shrink. However, the aluminum around it has a different coefficient of expansion so heat would probably expand the rubber more than the aluminum, causing it to be tighter when hot.
Either way, it is a suspension problem or the rea brakes are tending to have problem rotating, possibly the rear wheel bearings could be doing this, and when heated from driving spin more freely.
Check the rear brakes for signs of impeeded motion as well as the wheel bearings for rough spots or flat spots when rotated.
Then check the suspension for play or excessive travel. Also check the rear struts.