Imac G3 screen sometimes flickers and dies

[letawookiewin]

Hi there! I recently bought an old ruby Imac G3 off ebay. The only problem thus far is that every now and then the screen will begin to flicker drastically and sometimes completely blacken out. I'll then have to unplug the computer and turn it back on. It doesn't happen very much, but every now and then it does. This issue occurred before and after I installed an internal DVD drive(so I know the fact that I opened up the comp doesn't have anything to do with it). Also, I attached an external monitor and the flickering issue doesn't occur on the external when it does occur on the internal. I'd be fine with using the external for extended periods of time-especially to watch DVD's, however, when the screen on the internal dies after the drastic flicker, it causes the hard drive to sleep kinda-so the external display will disappear as well. I've heard of no ways to disable the internal monitor while enabling the external-so that's a no go as far as I know. Is it the logic board that needs to be replaced? I'm kind of new to all this computer stuff. Any suggestions or ideas? Thanks!