Imac monitor

My Imac, purchased in Feb. 99 has reacted to something.. but not sure it is serious or just what..at unexpected times, the monitor will shrink to an hourglass .. I thought it might be power changes.. (low power).. I checked my power plug and made sure it was a solid connection.. but can it mean the monitor is getting bad? Is there anyother thing to check? I have ran Norton Utilities etc.. no problem.. so??