Gonna need to see in the back where the reverse of the monitor is to work out if it is a 31KHz monitor chassis.
I'm pretty sure that desiv is correct. Change your resolution to one of the two mentioned and it should work. I'm not 100% familiar with Soft15KHz so I would have thought it would have done it for you but it's worth a look.
If you read the soft 15KHz guide
3. TIP: Change your desktop to 640x480 60Hz before restarting.