moniter problem for desktop pc

Well i just recently bought a dell dimension tower with lcd moniter. Everything was fine with it at first but after awhile the monitor started randomly freezing and go static with vertical and horizontal lines. When i turn it back on a square that says vga pops up. Ive replaced the monitor twice since then, but it still does it. Its pretty random and ive talked to dell techs about 3 times now, and it only fixes it temp. If you’ve ever had simular problems please message me back. thank you