Samsung HT-X40 System Problem. I have recently connected my home theatre to my Samsung 40' LCD via

Question

I have recently connected my home theatre to my Samsung 40' LCD via an optical cable so that I can use the surround sound while watching TV. However after about 10 to 15 mins I get a delay between the sound and the picture. The sound lags behind the picture on screen. This does not happen while I'm watching DVD on the system but I'd say that because I have a HDMI connection. Does anyone know why I get this delay??? Iv connected my samsung LE40R8 LCD tv to my samsung HT-X30 cinema system through toslink optical cable, at first the sound is good but after about 5 mins the sound runs out of sync with the picture seems to be a second behind!