Answer for:

Monitor Won't Stay In Standby

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OH Smeg

But it may be adversely affecting other things here. For instance a ATX Power Supply turns the computer on and off by having the On Switch produce a ripple small spike in the 5 V DC Rail of the Power Supply.

If the Power Supply is not the greatest a simple power spike could be causing it to produce a spike/ripple on another Voltage Rail or even starting the unit momentarily and then switching off again. When this happens the Monitor will start but as it doesn't get the signal to turn off it stays on.

As you can use a different computer and not have this issue I would be looking at the Power Supply in the computer that is causing this to happen.

Of course fitting a Filtering UPS between the computer/Monitor and Mains will most likely cure this as well. With some of the more modern Power Supplies they are built with corners cut and do not have enough Capacitance in them. They tend to work fine most of the time but can cause screwy problems that normally would not happen with a more expensive/better designed unit.

Does that answer how a Power Spike can be involved when this doesn't happen with a different computer?