monitor problem

I have a flat lcd dell monitor and my prob is when getting ready to use puter the monitor will work for only about 2-5 seconds at a time. I have to turn it off and back on for app 5-10 minutes before it finally stays on. Once it stays on it will work fine no matter how long I use it until I go a couple of hours of not using it, then I have to go through the on off thing again. Anybody got any ideas what it’s prob is?????? Thanks