My Phillips Televison

My Philllips television will not stay on for an extended period of time.  I turn it on and it will stay on for a few seconds then shuts off...the power light stays on but the screen goes blank.  I turn it off and back on and it will stay on a little longer and then do the same thing.  Sometimes it will stay on for a while ofther times I will have to turn it off and on 15-20 times before it stays on.  Any ideas???