I have not had this problem with my Hercules 8500 LE. I never used to have a monitor at all and just used the tv. Did you also get the latest ATI control panel? Make sure you have TV enabled and disable the monitor, go through all the settings to make sure your TV looks right (height, width, sharpness, etc). Also make sure its 800x640 resolution - then reboot with just tv connected. I'm sure you've already done this before but you might come across a setting you hadn't noticed before. Also make sure that the s-video cable is firmly plugged into the card and tv before turning on.
Like I said I never had this problem, in fact I was surprised when I turned on this pc for the first time ever and the boot screen loaded on the tv.
Good luck
-cdbrown