I understand now. The problem is that if i use smaller task like 1.0 or 0.5 then the fade in effect (that cannot be changed) is repeating at least 1 time per second and it makes illusion that text is blinking.
But if I set a task with longer time like 30 seconds then it's ok.
But when i try to clear synchud at item deploy and start a new message with modified string then it starts to overlap and text becomes white and messy.