Monitor Gets No Signal From Computer

I have a Dell Vostro 400 desktop and a Dell UltraSharp Digital Flat Panel Display (2208WFP) which are about 4 years old.  Computer is running Windows Vista Home Premium.  All of a sudden when I turn on the computer the computer boots up with no problem but the monitor says it is getting no signal from the computer.  If I turn off power to the computer and then restart it about 6 to 7 times after turning the power on and off to the computer, I finally get a signal to the monitor.  Unfortunately, I then get the message that Windows will have to do a repair since the computer was not shut down in the normal manner. 

I have checked all the cables, drivers and ran the Dell PC Checkup from the Dell Support Center program and everything has checked out.  No problems identified.  Is there anything I can do to fix this or can someone tell me what the problem could possibly be?