RichTextBox blinking

Hi! I've got an issue with RichTextBox. I'm formatting the text inside, indents, colours, font attributes etc. When I insert the text from the same control - it's ok. But when I do it from another one - my RTB blinks, as if repainting its contents. I've tried to change my text formatting, checked which events were fired and nothing. Any help from you? Thanx in advance.