Turning Off Monitor

Have enjoyed using WIN98SE for years. I set the monitor to shut off after 10 minutes of non-use. After 2 or 3 times of non-use the monitor does as instructed and turns off. After the 4th or 5th time the isnstruction to turn off the monitor stops. I then go back and reset the instruction and it works for another 2 or 3 times and then stops again. I have been doing this for the last 3 weeks and have no idea why the instruction to turn off the monitor becomes disabled. Can anyone please hellp. Leonard Dorfman