Internet explorer 6

I have windows XP pro, zone alarm , mcafee virus scan installed in my pc and a diamond supra E 56 pro modem . everytime i go on the net after 20 min of use i cannot display any page or download anything even if my internet connection is still connected.
I have checked settings, installed latest modem driver , still no clue why it stops working. I had to restart pc to use and again droped after 20 min of use.