I'm almost positive overclocking is software and not hardware, but I did previously overclock the card when it was in my newer system. Did I need to do anything to "unoverclock" it? I'm telling myself no, but just in case?
Just go into ccc and restore it to it's default settings then, especially if you aren't sure what it was at before. Overclocking may be the reason your psu isn't able to keep up this time, or in the 5 years the psu has already begun to degrade and you need a new one.