How do I stop flickering on the PC screen?

There are a variety of enemies that move about in different patterns. Contact with an enemy (or the missiles fired by some) will kill a dragon. The dragons' job is to complete the level by killing all enemies in it. If this is not achieved within a time limit, the message "Hurry up!" will flash across the screen. When this happens, enemies become "angry" (making them red and move faster thus making them more dangerous). Approximately ten seconds later, one or two Skel enemies appear on screen. Enemies also become "angry" if they escape from a bubble that is not burst quickly enough by one of the dragons. They may but not always calm down when one of the dragons dies. When in Super Mode, most enemies get swapped with a counterpart, e.g. PulPuls replace Monstas and viceversa.