The Mk IV is very sensitive to copied DVD's. It may be that the copy you have is marginal and is causing the drive head to move back and forth more often than it would with an original DVD due to read errors.

If you have a DVD burner then you could try making another copy of the copy, burning on a high quality DVD-R or +R, some DVD writers require a slow burn speed, others only work at high speed its a case of trial and error.

The other possibility is that the fan on your computer has failed, or just that the unit has got very hot in this weather.