Flat Panel Monitors - Monitor flickering on startup

Hello
I am using Compaq Presario, 2.93 gigahertz Intel Celeron and Windows XP Home Edition Service Pack 3 (build 2600). My monitor is Samsung SyncMaster 731B. It flickers very fast at start up for about 30 minutes and then if I push the power buton on and off repeatedly for around 10 minutes the flickering slows down until the monitor stays on. This process used to
take only about 5 min. I have looked at various threads online with few results, and have tried "troubleshoot" to no avail. Any help would be appreciated.
Thanks
Steve