Sudden frame drop

Jim, negativ not installed. Jamalje, with the mouse in the screen and off the screen. Well, not regular but ca. every 15-20 seconds, sometimes also already after 5 seconds. Really strange is that if i set the limiter (external or fs) to 20 the frame drops to 12, limited to 30 it drops to 20 and unlimited if stable at 55 down to 30, but with the lowest settings and on a small airport.??????? Noone else with this problem? Alexander Schönegger