connecting dell laptop to lg lcd tv - laptop keeps going blank

Hi.  Trying to connect dell laptop to LG LCD 32" TV via VGA/Serial port.  The connection initially works for about 10 seconds (resolution is perfect on TV) however then the laptop screen keeps going blank every 5 or so seconds (no signal msg comes up on TV)and sometimes returns with different resolution then goes blank again etc.
It is the same when i connect with S Video cable.
I changed refresh rate and resolution on laptop but it is still the same. Cheers