This is quite a wierd one: some time ago at a company I work for sometimes, a colleague tried to replace some old 15″ LCD monitors with shiny new 19″ ones, to be confronted by extereme flickering. I had a look, tried the monitors with my laptop, and got a flicker-free picture. I made sure the leads weren’t too close to mains cable, but no change.
We assumed some incompatibility with the (elderly) PCs, and another colleague changed the PCs recently. In the course of doing so, he discovered the real cause. A power lead- just a normal BS1363-IEC C13 (colloquially known as a kettle lead), but, tellingly, with a rewireable BS1363 plug, not a moulded one. Remove the lead, problem stops. This lead was connecting one of the PCs that was working perfectly well, and flicker-free with the 15″ monitor.
I looked at the lead the next day:
and it seemed kind of OK at a glance, though that neutral lead should have been cut shorter.
What did turn out to be wrong was every terminal was loose: loose enough to turn by hand, so I presume that the intermittent connection caused enough noise to upset the new monitor, but not the old one. Disturbingly, this lead had passed a current PAT test, when potentially it’s a fire hazard: loose connections can overheat.
I don’t know if the connections had worked loose (which is one reason why connections in screw terminals should not be tinned with solder) or just sloppily fitted in the first case. The plug did rattle when shaken, but it would do that even with tight terminals, as the pins have a bit of play in the housing. Full marks to my colleague for spotting an obscure fault.