we just purchased a magnavox star model number RK8536AK01...it is an older model tv and worked great when we got it....no when we can get it to turn on it plays for about 15-20 mins and the screen goes black.....we have tried turning it off and then turning it back on again but we only get the black screen....the green power light comes on and we can hear it turn on but we dont get any picture.....any help would be much appreciated...thanks