Laptop external monitor poor display quality

I'm using an old LG Flatron L1510S as external monitor for Toshiba Portege M800 laptop, with laptop's screen turned off. The problem is that flickering horizontal lines appear on the monitor (they are best visible against white background). I'm working in 1024x768 resolution with 70 Hz screen frequency, and changing these setings only makes the effect worse. Could anyone offer any advice? Thanks in advance.

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