Have you checked the wear on the rear tires to see if it's even across the tread width? I would have to say that it does look stiff for some reason. What gets me thinking is that it works when it's hot but not when it's cold. That could be a tire issue. Have you only used the M/T's or have you tried a softer compound in a different brand? Your car is pretty fast and most tracks I've run on would have a hard time holding that consistently.

Have you played with the pressure at all? Your videos don't show the burnout, maybe when it's cooler you need to do longer or shorter burnout. Lot's of stuff to try.