ACER AL1721 Monitor

I have 2 Acer AL1721 monitors which were purchased as faulty.
I had the inverter cards replaced in both screens as they kept going off and then coming back on; as though they were being power cycled.
One monitor appears to be okay now but the other still does this.
Does anyone have any idea what may be calling this??
Running Windows 2000 Pro with Matrox G450 with up to date drivers