Fine horizontal black lines in 19" Dell CRT

I bought a used Dell UltraScan P990 (These are re-branded Sony's, by my understanding), and after about 2 hours of use two fine black horizontal lines appeared on the monitor; one near the top and another near the bottom. They appeared while I was running the monitor at 1280x1024 @ 85 Hz, and I adjusted it down to 75 Hz just to be safe. Didn't fix it, though.

I am trying to get advice on how to diagnose and possibly repair this problem. The VGA connector has no bent pins, and I doubt it is EMI since I also have a Samsung 955DF and it is working beautifully (oh, so beautifully). I am planning on using the Samsung and selling the Dell to a friend, and I would like it to be in perfect working order.

P.S.: This is my first post in these forums, so feel free to flame!