Thinkpad monitor errors

I have an IBM Think Pad 770ED and since I bought it, I hve been able to use the computer for about 30 minutes before the screen goes blank. Then it appears to go into stand-by mode. The only way I can get the screen to come back on is to use the FN f7 buttons but then the screen continually blinks for approximately 45 seconds then goes blank again. I have tried changing the properties, I hve deleted the monitor from the hardware list in both the normal mode and the safe mode, but everytime it appears to solve the problem but after about 30 minutes it starts all over again. I am running W2K professional.