The login for my Pi is fine - recognises keyboard and mouse and power is fine. After I enter startx, a cursor appears and the Raspberry Pi desktop appears. But after a fraction of a second the desktop disappears and there's no HDMI signal. I then get a black screen, and then something like a screen saver (various different ones). It doesn't respond to keyboard or mouse at this stage.
PS Power LED is red, using power supply which came with the Pi