This can happen by a lot of things.

I inspected your code and saw that you update the text in the textbox regularly. It probably blinks because how the textboxes work in Winforms, every time you update the text it paints the whole thing again. For example:

int x = 0;
private void timer1_Tick(object sender, EventArgs e)
{
    textBox1.Text = "Textbox painted again: " + x;
    x++;
}

In this code we have a timer and the timers interval is 100. This means we update the text every 100 miliseconds. 100 miliseconds isn't a problem because it isn't too fast also we have just two lines of code inside the timer1_Tick function.

But I see 15+ lines in your timer and you didn't gave us information about the interval. If the interval is really low and with this lines blinking is normal.

The real solution here is simply optimising your code as much as possible and raise up the interval a bit, this will probably help. Do you really need all of that "if" statements? (I don't know because I don't know what the program does spesificly) Please let me now your timers interval and what this code does, than maybe I can find more solutions.