Auto-focus video problem

Hi,
I'm a casual photographer (occasional air displays, carnivals, weekend trips out etc),
I bought this Kodak Z915 digicam earlier this year £110/ 178 USD) and it blows my socks off, it's my first ever digicam, no more messing about with films, developing and printing for me..:)
My only real criticism is that it has auto-focus which is fine most of the time, but I wish there was a manual overide to allow manual focussing.
For example when using it to video something on a TV screen, the focus keeps blurring for about half a second at intervals of every 7 secs or so, as if the auto-focus is struggling to stay focussed.
Why should that be? Can't the auto-focus "lock onto" the TV screen or what? Perhaps the TV is giving out heat that confuses the sensor?
Any advice on overcoming it?
Thanks

http://i53.photobucket.com/albums/g6...b4/cam-pic.gif