>A friend said it happens when Directv puts too many customers on a transponder<
Your friend is an idiot, not to mention wrong.
Your Directv receiver is just a receiver and the situation is no different than a ground based TV transmitter sending out a signal to countless people with antennas on the house. There is absolutely no limit to the number of receivers which can receive a signal from a ground based transmitter or a satellite transponder (which is nothing more than a transmitter).
Now... to your problem.
Your TV has a display buffer in it and when the signal cuts out (or goes below the necessary signal to noise ratio at your receiver), the result is the picture on your screen freezes for a second and then probably pixalates before re-syncingg again.
Your problem is your signal strength at your antenna is too low. Do you know how to check your signal strength? I know it is under MENU but then I forget exactly how to ge there. It would be good for you to find that screen and check your signal strengths from several of the satelittes.
From your description, I'm going to tell you that you will see some numbers in the 60s and maybe even lower. Those numbers should be in the 80s and 90s.
if that is the case, as I said, your antenna is slightly misaimed.
Remember, those satelittes you are attempting to capture a signal from are over 25,000 miles away from you. A antenna that is mis aimmed by a fraction of a degree will cause exactly what you are seeing.