Just a thought after repairing my via card (thanks to youtube, the comp booted up then shut down , this happened a few times, after finally double checking all connectors again was fin for a week then on a reboot /start up all the fans kicked in , so let it load and did a shut down, the fault was the power lead , believe it or not, due to the initial problem heat was a problem, the main s lead had suffered , after replacing with my guitar amp plug line (common Kettle lead as i call it) fired straight up , the other issue was slow performance this is now gone, if you have poor power its obvious the rest will suffer, for under a fiver worth replacing , before panicking, seams to be quite common if you have had a previous problem , hope this helps, some one
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=== Update (02/26/2020) ===
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iMac 27" Vga card with lines and shuts down, look at the youtube oven trick it works!!!!, secondly this problem is over heating and can cause or the cause can be the power cable, i did the via fix and the screen was fine but had some shut downs, reboot fine and the again after checking everything again week later it shut down and was dead , the fault was the main power lead ( ie Kettle lead) had suffered heat inside , i tried my guitar amp power lead and fired straight up, only by chance did i come to this as had this with a marshall amp kept shutting down and over heating due to poor power supply conductivity hope this helps some one the cost under a fiver worth a try first.
Just a thought after repairing my via card (thanks to youtube, the comp booted up then shut down , this happened a few times, after finally double checking all connectors again was fin for a week then on a reboot /start up all the fans kicked in , so let it load and did a shut down, the fault was the power lead , believe it or not, due to the initial problem heat was a problem, the main s lead had suffered , after replacing with my guitar amp plug line (common Kettle lead as i call it) fired straight up , the other issue was slow performance this is now gone, if you have poor power its obvious the rest will suffer, for under a fiver worth replacing , before panicking, seams to be quite common if you have had a previous problem , hope this helps, some one