the timer interval is 1;
That is very, very bad. You are performing your calculation 1000 times a second. It's not that the ListView blinks EACH time. The problem is that you're doing it so frequently that the ListView doesn't even get a chance to repaint some times. Be realistic about the frequency with which you perform this calculation.

What are you calculating anyway? It may be that you don't even need a Timer at all but can instead use an event to recalculate only when the result may have changed. That's far more efficient.